I would say with no experience of my own besides one time making seeds of Moroccan beldia with 4 individuals… I would say 20 is a good number. Some people would say more or less. I think the more, the better for preserving.
Bodhi said this about Watermelon hashplant (18 plants)
You can see it continue some here with 3 different breeders F2s.
and about a smaller BSHB repro
“this seed run was an open pollination of 4 males and 6 females, not alot, but enough to get the job done… a small population open pollination in a tent means your going to mostly the beans made from the first two males that opened… the super quick sativa pheno male… and the fat indica dom male… this line is called big sur holy bud to differentiate it from the big sur holy weed lines by hhf, danbo, and reeferman. everybody that has tried this line has fallin in love with it. im really happy to gift this back to the community. very limited, only 30 packs, then its up to you to make more!”
So also think about what you want as open pollinating means the faster ones get most of the pollen. The later ones which are sometimes more desirable for more cannabinoid production, would be far less of a percentage. You can also do multiple continuous pollinations with some year round sativas as they reflower. I’ve seen people here do that.
You have enough to open pollinate and run lines you select for also.