if you have an intelligence of 5 by the time your level 30 how many skill could you max out

With no other help? Four. You could get one other skill to 97.

Consider investing in the Education perk - in terms of skill points/level, it effectively does the same job as increasing your intelligence by 4.

5-6 actually (3 are tagged so req half as many points invested)

First I've heard of it. In FO3 tagged = +15 points and that is it. I suspect, but am not sure, that you're confused.

This is not Fallout 1 or 2 rules fella.