How many command blocks can you power with a single redstone block?

With chain command blocks, infinitely many, since you can just set them up back-to-back, and powering the first one triggers the whole chain (assuming the rest are set to not need redstone).

Without chain command blocks, still infinitely many, because you can invert a redstone signal twice to refresh its power. This is more or less that a repeater does, if you think about it - it's crafted from two RS torches. Without a repeater, you can cheat and just put down two NOT gates :P

Without those, you still have a lot of options for extending a redstone signal like probably using hoppers and comparators or something silly like that; I'm not too sure, but the above definitely works.

For what you want to do, chain command blocks are... probably what you want?