From my experience (which might be incorrect) LLMs find hard time recognize how many words they will spit as response for a particular prompt. So I don't think this work in practice.

Indeed, it doesn't work. LLMs can't count. They have no need of how many words they've used. If you ask an LLM to track how many words or tokens it has used in a conversation, it will roleplay such counting with totally bullshit numbers.