There's 2 access programs to talk about. HMAs and WIAs.
On HMAs It varies by each individual HMA. Some HMAs are unlimited, you just have to apply for the slip. Others are very limited and it's like a lottery. And then there's some that have, say, 100 slips that are first come first served. You apply and if you're within the first 100 then you get one. That's the way I understand it at least.
WIAs are different. They provide access without having to obtain a permission slip. At least on the WIAs that I've looked at that's the case. Good luck, I hope I've helped with accurate information. If not someone please correct me!