These two very common words are frequently confused in everyday speech. Here is a helpful guide to help you avoid this confusion.
When we use 'many' or 'too many', the subject of our sentence should be plural. Anything that follows the word 'many' should be a number of things more than 1. Be careful with collective nouns, like 'stuff', for example. You should never say 'too many stuff', because 'stuff' is singular. We also don't say 'too much people' - we say 'too many people'.
You can also use 'many' or 'too many' at the end of a sentence. You do this when you are implying that you're talking about something plural. In the context of the conversation, people would already know what you mean.