I believe you really will only need 2 brushes to properly and beautifully apply your foundation.
1) The Sigma F80: Flat Kabuki:
The F80 is designed to blend out and apply liquid or cream foundations. It provides such a flawless application I literally put in half the time blending out my foundation with this than I did before owning it. It has a flat head which is one of the many wonderful things about this brush. Comparing it to a stippling brush I cut my application time in half. I somehow always spend way more time than I need working on blending it out when using a stippling brush because they just are never dense enough for me. This brush is $21 and way, way worth the investment. I use mine almost every day–with exception only when I’m using the brush I will talk about next.
2) The Real Technique: Expert Face Brush:
This my go-to brush when my F80 is dirty or when I only have the time to take 30 seconds to apply my foundation. I used this brush this morning and my makeup looked smooth and had no lines from the application. I don’t have time to touch up my makeup at all during the day working with kids so not needing to worry about how my foundation looks is way up on my list of priorities. Just like the F80 it has a very dense head and is known for its ability to make your skin look flawless and essentially near perfect. This is more a more cost efficient brush at only $9. If you don’t have much to spend on a foundation brush and want one you can trust I’d buy this one.