I think your understanding of necessary assumptions is correct. The use of "valid" may have been a little confusing. Here's an explanation anyways:

Often times I find necessary assumptions don't make an argument valid.

They don't; necessary assumptions HAVE to be true for the argument to hold, but they don't necessarily make the argument valid. That's sufficient assumption. And you're exactly right, there's a billion necessary assumptions for many arguments but the LSAT will only have one as the correct answer. Here's a very simple (and stupid) example: If I have a license and it's not suspended, then my car is green. A necessary assumption is that I own a car. Owning a car doesn't make my argument valid (there could be many other reasons why my car isn't green), but if I didn't own a car, then my argument falls apart and it's no longer valid. Hope that makes sense! Sorry for the stupid example, it's late and my brain refuses to think of something better.