I've never tried this for that many people, but I do regularly make up a quart or so of tea for drinking when I'm away from my tea stuff. I use a 6 oz or so teapot or my kamjove infuser, with enough leaf to pretty much stuff it full when the leaf is opened fully, and brew several infusions that I mix together in my thermos. This works very well for some teas, not as well for others, because tea continues to oxidize while being held as liquid in the thermos.
So if you're drinking it quickly enough, and the thermos is clean, a delicate white or green tea will still taste pretty good, but if held too long, they start to taste like a black tea. For longer holding, the ones that work best for me are puerh, and darker roast oolongs.
Depending on your goals here, you might start infusing one of these sturdier teas just before your guests start to arrive, to hold in a thermos and have ready when they walk in, and then work with more delicate teas that you brew continuously during the party, and share in smaller sipping cups with a few people at a time.
And remember that all you really need to brew up some tea is a vessel, and leaves. I have brewed up a larger quantity of tea in a heatproof bowl or measuring cup or pitcher, poured all through a simple strainer to catch the leaves, and poured that into my thermos--find a pint or quart pitcher and a strainer, they don't have to be fancy to work for this, and you can brew in bulk. You'll need to really look and smell the tea as it brews to be sure you get a nice batch when you're not used to brewing in this quantity, but it can work great.