There are only two different types of maintenance exams. If you look at any of the release training, you will find this on the bottom:
Note – If you are a Salesforce.com Certified Advanced Administrator,
Service Cloud Consultant, or Sales Cloud Consultant, you must complete
the Certified Administrator exam. If you are a Salesforce.com
Certified Advanced Developer or a Salesforce.com Certified Technical
Architect, you must take the Certified Force.com Developer exam.
So, this breaks down like:
Admin Maintenance Exam covers:
- Salesforce.com Certified Administrator
- Salesforce.com Certified Advanced Administrator
- Salesforce.com Certified Service Cloud Consultant
- Salesforce.com Certified Sales Cloud Consultant
Developer Maintenance Exam covers:
- Salesforce.com Certified Developer
- Salesforce.com Certified Advanced Developer
- Salesforce.com Certified Technical Architect
So, if you have every certification, taking just two maintenance exams will cover you for all of them.
As far as studying for it goes, just go over the material they provide in the Learning Center. Take Summer '12 for instance, notice how they break down exactly which videos to watch. If you watch those videos, I guarantee you will pass. The tests are generally very simply (5-15 questions) and are meant to keep your knowledge of the platform up-to-date. You pay $100 every 3 releases (aka $100 a year).
If for some reason you do fail a maintenance exam, you get another 2 more tries for free. If you can not pass the maintenance exam in 3 tries, well, you really don't deserve to keep the cert at that point. Good luck, but it really isn't very difficult.