Bicycle Tutor

I'm buying a set of spoke nipple wrenches, but I'm not sure which sizes I'll need. For what it's worth, I mostly work on basic, Walmart-level mountain bikes. Thanks!

There are three common sizes. They are typically marked red, green, and black in the Park and similar tools. Red and green are more common on cheaper bikes. Black is more common on higher end wheels.
You can get the multi-size tools that fit everything. But they tend to be less well made and it is nicer working with a single tool of the right size. But up to your budget.