If you're printing on demand with transfers then just keep a minimum amount of shirts in stock; a couple per size/color until you see what sells. After all, you can get more blanks in a day or two.
My question is when I am ready to launch my website and start selling, should I have a certain number of blank shirts in stock, how many? different colors and sizes?QUOTE]
These are questions you will have to play around with, find out and answer for yourself. We really can't tell you how much you'll personally need and what colors you should get. Everyone is different...with niche markets, income, goals, etc....
For myself...I went with screen printing. I made 6 designs, a few with different colors and 3-4 of each size (S-XL). Now the downside to that is...If you do not sell, you'll have a bunch of inventory left over...which I understood and gambled with.