1. Submit the sooner, the better. Ideally, you should submit before your MCAT, but you may be busy preparing for your MCAT, so you won't be able to submit until afterward.
2. Specific requirements differ by school. Look through each school's website for specific requirements. I think 2 science LORs, 1 non-science LOR and 1 character reference should cover the basics.
3. There is no difference applying to one school versus 25, except for cost. All you have to do is click "add school" and "assign LORs" to that school. If you're going to be going through ALL the trouble of putting together an AMCAS application, you might as well apply to more schools. Being a Canadian, I would advise 10+ (apply broadly and apply smart).
4. Secondary applications vary between schools. Some schools have 3 essays that you have to write, some schools have none. Finish secondaries by June-July to be considered early. August-September is ok. Past September is late.
The answers to all your questions can be found through the search function...so perhaps try searching before posting in the future?
Good luck!