Theoretically, none... you just need to be able to wrap a rope around it and it needs to be near the Center of Gravity. Doesn't even need to be eyelets or anything fancy, it could just be some holes in your frame at the right place. As long as it's accessible and easy to attach a rope to, etc..
Quote:
R10:
Teams must supply at least two (2) attachment points for the belaying device (see Section 2.2.5) to mount to their ROBOTS. Attachment points must be:
A. easily accessible after the ROBOT has CLIMBED the PYRAMID,
B. on opposite sides of the ROBOT,
C. located near the ROBOT’S balance point, and
D. made from exposed structural members that will allow a rope to be wrapped around it or two eyelets (McMaster PN3014T45 or similar) mounted to the frame. Opening of the eyelets must be at least ¾ in. in diameter.
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http://frc-manual.usfirst.org/viewItem/279