a little while later his dad starts asking so can you confirm will there be 2 or 3 nans.
FuckSAKE competitive grandparenting already.
Charchar, don't allow yourself or DH to seethe on this - you need to get it out there pronto, or it will be the thin end of the wedge with
FiL annexing what he perceives as his decision-making authority & blaming you when you disagree about anything to do with HIS grandchild (i.e. not - note - YOUR baby.)
You only have to endure him 3 or 4 times a year, so woman & man up.
"FiL/Dad - it's not actually about you, it's OUR baby & we're the ones making the decisions. You will be grandpa, MiL/Mum will be grandma & [your wife] will be step-grandma."
Or whatever names/titles you & DH re happy about & want the seniors to run with.
If you get pushback:
"does it matter? Is the name the baby is ging to be using for you actually more important than the relationship you are going to be starting with him/her? - because this feels like politics, FiL/Dad - & we're not having political discussions about our baby."
Then instantly move on to bright remarks about the weather, the garden, fancying a cup of tea - whatever.
Just shut this shit down. It's poison, you can choose not to drink it.