ramrajone 1.11 (and thus 1.12) have quite a lot of new features that aren't in 1.10, most of which are listed here: https://docs.osticket.com/en/latest/Getting%20Started/v1-11%20Overview.html

You should be able to upgrade directly to 1.12 from 1.10.4, although make sure to back up everything first and put the site into offline mode before starting.

ramrajone Second question was answered in my first sentence ? 1.12 has a whole load of new features first introduced in 1.11, and if you're going to upgrade anyway, might as well go to 1.12. Chances are 1.10 will fall off the support list sooner than 1.12 anyway

You should be able to go from 1.6ST to any version of osTicket. So you could go from 1.6ST to 1.12 if you want/need to. The short version is that you can go from 1.10 to 1.12.