Traditionally dissertations are between 225 pages and 275 pages (while they can be longer, don't do this). Chapter numbers vary, but Introduction, Conclusion, and then 3-4 middle chapters is pretty common.
Here is the registrar's official page detailing formatting and submission requirements for the dissertation. This page contains a sample dissertation illustrating Harvard's specific formatting rules, which is especially helpful.
There are many websites and books that give advice, exercises, plans and more for dissertation writing. Here are some! The Thesis Whisperer. It has fun entries on things like, Thesis Prison and The Valley of Shit.
Writing your dissertation can be very isolating, so we have started a Dissertation writers' workshop.
Don't be afraid to contact your supervisors and make/request regularly scheduled appointments with them. They will not do this for you, and just because they aren't doesn't mean they won't meet with you.
Please feel free to add advice, pointers, etc. here!