por and para

Is there a way to know when to use “por” for for and “para” for for?

For example “For how many people"  is Para cuantas personas  but "For how many nights is Por cuantas noches.

I keep getting it wrong so was wondering it someone had a little tip I could use.

It is definitely confusing. The good news is either way, a native speaker will understand you (it will just sound strange).

Por vs Para is covered in lesson 11.6 and it is complex. And you have to have purchased Level 2

The rule of thumb I use is if it looks to the past (the cause) use por. If you are talking about the future (the effect), use para. It is far from perfect, but carries you most of the way.

Thanks thats a great help