MIT says "There's no hard limit on the number of blocks allowed in a screen. However, the more blocks you have the more time your browser needs to spend rendering them, and the more RAM is needed to keep the project in memory while you're coding it. There's also some issues with really complex sequences of blocks triggering out of memory errors when building apps, so if you start to encounter failures when building you might need to reduce the block count."
Based on comments in the Forum regarding maximum blocks; developers start seeing serious issues when their block count exceeds around 7000 blocks.
They experience slow rendering of new Blocks on the Block screen, compiling issues etc. What is maximum depends on your hardware, browser and Internet connection. When things start to not work; you have reached your limit.