It differs between different Senchas. For temperature, usually between 165° and 178° for the first infusion. Multiple infusions are definitely possible. First steeping should be just under 1:30m, followed by 30s, 60s, 3m. These times should be loosely followed as it greatly differs depending on the Sencha. You'll also need to start bumping up the temperature after the second infusion. Amount of leaf depends on what you like. I use about 1.3tsp/100ml, some use more, some use less.
Remember that Sencha should not be bitter, if you get a bitter infusion, then first try decreasing the temperature, then go ahead and decrease the time as well.
Every different type of Sencha you get will need to be steeped differently. It will really fall onto you to find out what each Sencha needs.