Return ticket mandate for B2 visa?

My mother in law and father in law are coming US this month. I have booked only one way ticket and planning to book their return ticket once they are here.

Is it mandate to carry Return ticket also? They say it is good to carry if not they might reduce the I94 and we cannot extend it.

I would like them to stay here for a minimum of five to six months.

Your help is very much appreciated!!! thanks…

This is one of those ‘what kind of mood the CBP officer is in?’ kind of questions. Just book something so they can carry and cancel it depending on the I94