Is there a limit to how many blocks I can use?
There's no set limit to the number of blocks you can add. But don't make your levels larger than necessary. More blocks means less performance, and not everyone has a high-end phone to play your game on.
There is a limit to the number of custom block types you can create in a single game. The exact number depends on technical details, but the same principle applies, don't make more than necessary. Can the same block be used again? Can you split a multi-block and reuse parts of it? Can you make your game on a smaller scale?