I use a 1mm drill bit and do about 16 - 24 holes for a 2.5 or 5 litre bucket. If you stack the lids up tightly you can go through several at a time.
The secret to use is to not fill to the top, hold bucket lid up, squeeze top to bottom to push some air out, and whilst holding the squeeze (as it were) turn the bucket upside down and relax. The vacuum holds the syrup back, but clearly you want a secure lid!