Really comes down to what kind of space you have, just like a normal plant. Bigger the pot, bigger the plant(generally). Generally for a mother plant though you want a decent sized space as with it being kept in veg it’s going to keep growing until you either flip it or it dies. Whatever you do make sure to keep those roots healthy!

Id say 5-10 gallon.

I don't keep a mother for very long. I veg in 1 gallon pots. When the plant is ready take your cuts. Wait a couple of days so the
plant can recover then up pot to whatever size you flower in. In 10-14 days you'll know if the cuts took and then you can flower
that plant or take more cuts.

This method keeps everything healthy and plants aren't around long enough to get root bound.

As ohiogrow said, it all depends on your space and what you want to accomplish. If I needed 30 or more cuts or wanted to keep
a mother for a long period then go bigger like Glomus suggested. There is no wrong answer.