You need repeating pattern due to how steppers work. Theoretically you can have different chips, but they will be distributed evenly.

And "yield" in this case is not how many chips can be placed on a wafer, but how many of them will not have defects in critical areas.

Thanks!

There are foundries that do multi-project wafer runs for very small customers. It's not motivated by yields, though, but by sharing fixed costs with other customers if you don't really want to do mass production.

Plus customers pay per wafer, so you have put different customers entirely.

TSMC themselves don't care how many chips gets made from a single wafer.