Platemates are actually not recommended for use with kettlebells as they change the dynamics of the kettlebell. They are designed the way they are for a reason.
If you're just starting out with kettlebells, a 15 is a good starting point as the exercises that require a lighter weight can be done with dumbbells until you get the move down (a lot of the moves require technique more than strength - TGU's and Windmills included). But if you already have a 10, then I would go to a 20.
As for doubles - of all of my videos, I only have 2 at this point that require doubles - (AOS Firepower does - but not Providence or Newport, and Cotter's Extreme Kettlebells does for some of the heavy lifting), although the Kettle Goddess does offer some variations that use 2. One is plenty to start - you can add more later if you decide this is the training for you.
