It will depend on where you live. In NC, the local inspector can sign off himself on any 12" thick block wall up to 9' high. After that, he demands a set of plans with an engineers stamp. As long as an engineer signs it, you can build anything you want. He will pass it as long as the finished product MATCHES the plans. However, there is eventually going to be some type of limit as to how much rebar you can put in a wall and still have it safe. Expect a high steel bill if you go above 10'. Plus you will have to fill all the cavities once the steel is in place and inspected.
I would guess than 12" block could easily go 30', since you see big box stores all the time that high. What are you building where extreme height might be a concern?
Needless to say, block walls over 10' are not for amateurs to build.