No.of holes per round - 18
Since a Hole-in-one is only possible on a par 3, and there are generally only four par 3's per regulation course, you should adjust that factor from 18 to 4.
On the other hand, many courses are par 3 only. So probably a realistic factor would be "6" or thereabouts.
Actually, there are a few recorded holes in one on par 4's.
Not true. They do happen on Par 4s from time to time. There were several on the PGA Tour last year and at least one that I know of this year. Some courses have a very short par 4 or several on them. Ely Park in Binghamton NC is such a course. I can drive all but 3 of the par 4s. Several of them with an iron.
and there are generally only four par 3's per regulation course
There is no such thing as a "regulation" course. Just look at the variety on the PGA Tour. You have par 70, 71, and 72. What determines Par is the number of par 3s and par 5s. If they are equal you will have a par 72. If they are not you will not.