Doesn't appear to be a limit on how many plugs you can have.
The number of holes allowed is 12.
USBC Equipment Specifications and Certification Manual
Plugs and Designs/Logos:
1. Plugs may be inserted for the purpose of redrilling
the ball.
2. Designs may be imbedded in the ball for guides,
observation, or identification purposes, provided such
designs are flush with the outer surface of the ball.
3. In all cases there shall be no interior voids.
4. Plugs and designs must be made of material similar to,
although not exactly the same as the original material
of which the ball was made, and shall otherwise
comply with all other specifications for a bowling ball.
5. Density not to exceed 1.5 grams per cubic centimeter.
Holes
Holes:
The following limitations shall govern the drilling of
holes in the ball:
1. Holes or indentations for gripping purposes shall not
exceed five (5) and shall be limited to one for each
finger and one for the thumb, all for the same hand.
The player is not required to use all the holes in any
specific delivery, but they must be able to demonstrate,
with the same hand, that each hole can be used
simultaneously for gripping purposes. Any hole that
cannot be reasonably shown to be used with a single
hand would be classified as a balance hole.
2. One hole for balance purposes not to exceed 1- 1/4
inches diameter. This hole may not exceed 1- ¼
inches at any point through the depth of the hole.
3. No more than one vent hole to each finger and/or
thumb hole not to exceed ¼ inch in diameter. USBC
considers a vent hole any non gripping hole that
intersects with a gripping hole at any depth. Any
hole intended for use as a balance hole that intersects
with a gripping hole will instantly be considered
a vent hole. This hole may not exceed ¼ inch
at any point throughout the depth of the hole.
4. One mill hole for inspection purposes not to exceed
5/8 inch in diameter and 1/8 inch in depth.
