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All depends on you personal build (precision pressing threaded inserts, motor type...) . On mine mostly are 5mm but the botom ones. It is not an exact number.

Basic build maybe:
16pcs M3x5 for the top plate
16pcs M3x5 for the motors
8pcs M3x8 for the botom and legs plate.
4Pcs M3x8 for the power distribution board.

Washers are optional, i use them only on the motors, so i use 16pcs M3 washers.

The best is to assembling one and then go for the rest of parts.