How to decide how much product to have on hand?

Wow, so busy life hits us hard. I think this is normal for me because I expect things to happen faster/more efficiently than they do. So, other than being busy with sewing projects/work, I'm happy to say that I sold my first pair of gauges for the year. The gorgeous vintage Magnolias found a new home. 

Such a happy day for any creator.

I think I will wait till I sell one more pair of gauges and then make another 4 sets. I know I need more selection. How would you choose how many to make and what styles?