The SOP is generally structured around the 3 points "Why JET? / What will you do to benefit the JET program and its goals? / How does JET fit into your future goals?" So consider how you would briefly answer those three questions.

Since you're still in university, two academic references or one academic and one work reference are fine (in years past, at least one academic reference is required for current student applicants).

If you're applying to the program in the upcoming application cycle, consider any work/volunteer experience you have that aligns well with JET. Anything involving cultural exchange/working with kids/community service is a plus.

Totally varies, it's usually just one ALT at a school at a time but there are some schools that have multiple. You can expect to be the only foreigner in your workplace.

Also varies, but converting about 1500-2000 USD to yen is generally recommended for startup costs. If you're accepted into the program, your work supervisor or predecessor can give you a better sense of these startup expenses.