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?