If say 300 wafers are made with 1000 different designs on it, everyone gets 300ish chips

That's not how it works. The same pattern, called a "reticule" [1] is duplicated 300 times over the wafer. One wafer gives you 300 copies of whatever set of dies fit into one reticule.

[1] it's to do with the size of the optical/UV lenses that are used in mask-making