I only did 4 FL's and felt completely prepared for my exam. I don't remember which ones except that I know I did 3 and 11 and they were all from the AAMC.
I see FL's as a way to test your ability to get used to taking such a long test. I don't think you need very many practice tests for that purpose. When I see people taking 15+ practice tests, I think to myself "what is the point of that?". It seems especially pointless to me when they start taking FL's before they finish content review. If you are not solid on content, then there is no way to know if your low score on your practice exam is due to lacking content or poor test taking skills.
If you want to take them to identify weaknesses, I think that is a waste of time and FL's. You should be able to do identify weaknesses pretty easily as you work though the content of whatever books you are using(EK 1001 and TBR passages work great). If you are on the nitrogen compounds section of organic chemistry and you do terribly and were lost on the passages, you know it is a weakness.
Another good way to find weaknesses is to go to the MCAT Q&A section because many of the problems others have are common problems for people and thus likely to be something you will want another look at.
If you want them for practice with problems, the TBR books do a great job of offering well written practice problems and you get a bunch on the same topic which is great for driving that topic in. There are so many that you can do a bunch right after reading the chapter, and then come back to the section a few times after that for practice. EK 1001 is also a great resource for that with its plethora of problems although their biggest benefit is that they quickly show you where your content is lacking because they purely test your knowledge of a topic, not how well you read a passage.
If you want them for content review or to figure out what might be on the exam, that is just silly since that is what the books and the topic lists provided by the AAMC are for.
If you are having trouble finishing the exam and are certain that you do not have issues with content, they could be useful, but only if you get some help developing a decent test taking strategy and then practice it with them. I would hope that you don't need to buy all of the TBR, EK, AAMC, etc. tests to find a good test strategy though.
