You need to treat the all snakes and all the equipment. How many treatments depends in what you use. If it is a weekly treatment I would do three myself. With the right product done properly the first treatment should do it, you just do 2 more to be safe. I use a Nix solution. Some people don't like it because it's a product that is for humans but when used right it works amazing and I stand by it.

Make sure you use. Product that kills eggs a well as the mites themselves. Biggest thing is really do the treatment properly. If you cut corners they will come back.

The one with the lots in it could have been the original and most likely is but not neccisarily. It's also right for you to treat all the snakes. Just because the others seem like they don't have any it most likely just means it hasn't gottin to serious yet. Generally when you have mite issues you won't even see them just crawling around. It's really bad at that point. For every mite you see there are dozens more you don't.

Sounds like you are on the right track. Keep it up.