Is it possible to create git repository in git.kernel.org for Microblaze
cpu?
You need to ask the kernel.org administrators for that:
http://www.kernel.org/faq/#account
How many maintainers are acceptable? (one or more)
You can have as many maintainers as you want but you probably don't
want to make it too many. There aren't any "official" responsibilities
as a maintainer, it really depends on how much time and effort you're
willing to put in.
You don't necessarily need a public git repository before sending your
patches to LKML but yeah, you definitely want some public exposure
before asking for Linus/Andrew to merge your port.