This is really impossible to tell. It all depends what you are going to store on your iPad. Lots of people would be fine with 32 GB, others run out of space on 512 GB.
My phone has 128 GB. It holds: Lots of music. Lots of audio books. Lots of videos. (That's the big parts). 16 years worth of photos (that's not really that much, mostly because the first years I used a 2 MP camera). If you intend to take lots and lots of high resolution photos, go for 256 GB. If you intend to take lots and lots of high resolution videos, go for 512 GB. Now it depends what you and I understand when we say "lots and lots"
If you have an iPad already: If more than 80 GB are used now, then you will run out of space with 128 GB eventually, so go for 256 GB. For example secretK with 198 GB on her current iPad should go for 512 GB on her next one - in my opinion.