Humbly, I decline to give my answer behind closed doors, I know an enormous amount more than I did 4 years ago, but still know relatively little, so am starting this thread for a variety of input.
Various commentators remark on advice and guidance they have gleaned from their 'Teamaster'. It seems comparing these 'rules' that there are great discrepancies, but often they are comparing tea from different regions - Anxi, Fenghuang, Taiwan and Wuyi. In addition to Region many other differences come into play:The local water; the level of oxidisation; the level of roasting; the shape of the processed leaves and the age of the tea. In many cases the so named 'Teamaster' may be playing with relatively few of these variables and may have different personal preferences to some of us.
I think the combined weight of all teachatters will provide a more comprehensive recommendation than any single teamaster, as long as you can sift what you want from the answers.
There are also 'Claymasters' who know their yixing clays and may be willing to go to excessive lengths to match clays and teas. There also exist other 'Claymasters' who know the wonderful benefits of Banko, Shigaraki and Gaiwans. I do not believe this chatter wants advice on anything other than Yixing, so the usual chorus of 'Just buy 1 gaiwan' would be wasted. I also feel from what little I know of this teachatter that they won't be shy of buying several, even many, good quality yixing pots over the next year.
The problem I see is size, shape, thickness, clay and tea regions - how many variations do you need?
1. Size
Unless you are of Northern Chinese extraction and need 300ml to 500ml pots for 'family brewing' anything over 200ml should probably be rule out.
50 - 100ml are favoured by diehard Oolong (and pu) fans and have the advantages that you can use less leaf to get a heavy brew, you can more frequently extract every last drop of goodness from the leaf at 1 session, and if you have more than 1 you can manage side by side comparitive tasting easily.
150 - 200ml are great for group tastings where several of you can get a decent sample from each infusions, and are also great for leisurely infusions when you don't want to keep running back and forth ftrom brewing station to drinking location. It can be great for large gatherings but can be wasteful on leaf when drinking alone.
100 - 150ml is in someways a good compromise, good to share infusions with just one or 2 companions, the openings will usually give easy access to all leaf shapes, and if you are a thirsty drinker can give you plenty to drink when having solo sessions.
2. Shape
Countless variations, but perhaps can be split into 3 major types; flatter, rounder and taller. I have read that many claim the shape affects the tea tatse but am in no position to concur. I do believe that shape can be important for leaf type.
Flatter ones can usually accomodate better the long twisty leaves of certain Wuyi Yan Cha. Perhaps Fang Gu!
Rounder ones can better accomodate the pellet type leaf from Anxi and Taiwan, allowing the leaves to unfurl and decent extraction in a uniform way. Perhaps Long Dan, Xishi or others.
Taller ones - I don't know - I read once or twice but have no recall.
Compromise shapes may include Shi Pao and Shui Pin.
3. Thickness
Along with the heat retention properties of the clay, the thickness is supposed to have some bearing on the finished infusion. Though I have read so many conflicting ideas that I have not reached a favoured conclusion. But if you want for a kind of Oolong, initial very high temperature that cools off fairly rapidly long before you decant then it may be worth looking for a thin walled yixing. [others claim that even thin walled pots of certain clays retain heat well - so I am a little lost here]Thick walled are supposed to retain heat extra well, and for other Oolongs you may want a thick walled pot which helps you with extra long infusions.
In practice I think many reasonable value available yixings are medium walled - thinking how they are made from slabs of clay rather than thrown, thick walled cheap ones from clumsy inexperience potters, medium walled pots from those who have acquired greater skill, thin walled only from true masters [last sentence somewhat tongue in cheek]
4. Clay
Perhaps the most interesting part, some teapots are simply designated Zisha or Purple Clay - how much this conveys I don't know for other pots are given intricate clay descriptions not only identifying type of clay, but sub-type and source location.
Some will settle for (or even insist on) Zhuni on every single pot in their Yixing collection. Others will claim modern zhuni is not so good, and will identify other clays for a subrange of their oolongs.
Some clays are more absorptive others have better heat retaining properties. The more absorptive the pots you buy, then the more likely you are to want a larger collection initially these pots may rob you of some flavour from the tea, but once well seasoned with a particular tea type you don't want it throwing that back into another Oolong.
5 Regions
a) wuyi yan cha
I have seen that some teachatters split 2 yixings between normal and aged. I have seen other teachatters split 2 between high roast and light roast, and some that split into 3 and others that split into certain varietals. I am hoping to buy an extra pot especially for Bai Ji Guan.
For a smaller pot you may wish to give concern to the size of the lid opening.
b) anxi
There are some who want Zhu Ni but others who want a softer clay like Duanni and Lu Ni. The round shape for the pot is worth considering.
Greener TKY would probably demand a seperate pot from wuyi, but if your taste is for traditional highly oxidised TGY then maybe a common pot for wuyi and TKY would suffice.
c) taiwan
don't ask me, I have not explored many taiwan teas I use them (dongdings and alishans) in the same yellow clay teapots I use for Anxi. BaoZhongs or Pouchongs I use a seperate very cheap zen8teapot that is blue.
d) Dan Cong
I still use some unnamed brownish purple clay pots (with wide openings) that brew well before deciding where to finally allocate. I now have Chao Zhou teapots for my Dan Congs so would not worry too much about yixing. Maybe a general purpose yixing to help you on your way, but one that does not retain the heat too long (or astringent flavours will come to the fore) Though again I have heard some claim the extremely thin walled Chao Zhou retain heat well, I don't think I believe it - if the heat of the exterior is so obviously very hot almost immediately then it is transferring the heat into contact with the air and the interior must be cooling.
I think you should start with a couple of mid priced pots in your preferred size that you are prepared to NOT dedicate any further than splitting the whole of the Oolong kingdom into 2
1 for wuyi and dan cong maybe 120 ml
1 for Greener pellet shaped Anxi and Taiwan maybe a little larger like 180ml to allow the leaf to unfurl completely
Play with them, continue to observe advice and guidance on clays and teas. Then when you have decided which Oolong types you REALLY like a lot then move on to getting dedicated pots for them.
Just 2
But knowing you, I hazard a guess you will own 12 within 4 months
