Tim, no sir, you cannot choose between 10 & 13. This gun comes with 2 magazines, (1) 10rd and (1) 13rd magazine.
It comes like that in the box, you don't get to choose two 10rd mags or two 13rd mags. Bud's does not alter the package the way it comes from the factory, except to remove non-compliant magazines.
I hope this clears that up for you.

Hi Tim and welcome. That verbiage says that you can choose if I am reading it correctly. I would contact Buds Customer Service directly for clarification at 1-859-368-0371 9am-9pm EST Mon-Fri. or use the Blue Chat now button at the bottom right corner 9am to 9pm Mon - Sat and 9am to 7pm Sunday.

I looked on the item page and couldn't find where it gave you that option?
This model comes straight from the factory with 2 magazines. (1)-10 round & (1)-13 round. Unfortunately, there's no choosing. If your state has a magazine capacity of 10, then Bud's will REMOVE the 13 round magazine & will Not replace it with another 10 round magazine.