How far apart can I install rectangular undereave vents? What size hole is required for the undereave vents?

The more undereave vents that can be installed the better in all cases.

The installation of undereave vents should always be focused on trying to achieve a consistent flow of air along the entire underside of the roof deck. This is best accomplished with continuous intake venting. Therefore, the more undereave vents that can be installed the better in all cases.  Cathedral ceilings would require at least one undereave vent between every pair of rafters.  For open, flat ceiling attics it is recommended to install undereave vents every 2 to 4 apart or  better yet -- in every rafter bay if possible.

Cutting the holes for undereave vents is similar to cutting the slot opening for a ridge vent: the restriction should not be the hole it should be the vent itself. Therefore, the hole size should be close to the actual size of the undereave vent but not so large that the vent cannot be fastened to the soffit.