Can you say “much more people?”
No - it is not grammatically correct or ever used. So either Many more people came to the party last time. or Far more people came to the party last time. Both are correct, and commonly used.
Edited to add further to: to explain a little more about when you would use "far more" and when you would use "many more". Are "many" and "far" both used interchangeably. It is a question of degree mostly, with "far more" normally implying a much bigger increase than "many more" (in the context of countable items). I don't think there are any firm rules about it.
And in terms of adjectival use, both "far" and "much" are used: "I am far more comfortable ..." or "I am much more comfortable ...".
And also note that the opposite is fewer (used with countable items, like people or pencils) rather than less (used with indivisible quantities, such as water or paper). The more usual usage would be far fewer rather than many fewer.