Some of the following may not be strictly accurate. Keygrip is the one to ask, however...
1. There is a single, multi-choice exam for each certificate. They're done on demand at just about every flying school in the country & marked immediately.
There are a number of self study books for all the FAA tests. All questions in the FAA database are available for public scrutiny courtesy of the FoI laws and most exam prep books will have each & every question in them that pertains to exam they're covering. The texts will also give the options (part of the questions so still an FoI item) and, crucially, which one is correct & why. The author's idea of the correct answer & reasoning is what you're paying for, although the books aren't expensive.
Gleim, ASA & Jeppesen are organisations that cover most things.
You'll also need the Practical Test Standard for the flight tests you'll be doing. They detail what you must know & do for the certificate(s). Also available from the above publishers.
All available at any pilot shop, most FBOs (Fixed Base Operators - think combination flying school, maintenance, fueling, handling, briefing office, etc etc) and on the net.
2. No formal ground school required. You *may* need a sign off from an instructor. Not sure about that point.
3. One exam per certificate type (think licence or rating). Pass mark typically 75% (?).
4. Have to ask a flying school I susect. Or the FAA - they keep records on licenced school's & individual instructor's success/fail rate.
4. It can be. Depends on how well prepared you are. Expect to be given application level questions about the things you must know & do to exercise the privileges of the certificate you're being tested for. As if you already held the licence/rating & this is just another flight using it with the sorts of things that you'll have to deal with in normal use. There will also be a/c & operational limitations + probably whatever is the FAA's & the testing officer's current safety initiative.