There are a bunch of rules I seem to remember about ability score maximums, but I can't remember where they come from or if they're actually the rules as written:
I believe there is a RAW that you can't exceed 18 on any of those either at creation or by buying points every four levels. For example I started with DEX 15, bought new points in DEX at levels 4th, 8th, 12th, and as for level 16th the cap of 18 will already be reached and I will have to improve another characteristic instead. Did not mention it but of course, racial bonuses if applicable can shift up the cap.
I believe there is also a RAW that you can bypass the cap by using a maximum of 5 tomes, making the real cap at 23.
Finally, for a standard human who bought 5 tomes and wears gloves of dexterity +6, the hard limit for its dexterity would be 29. (I purposely did not pick the example of strength because I believe there would be some way around it. Also in my universe, I have a way of getting one more dexterity point and reaching 30 but that is not the matter).
Are all those statements corrects, and (if so) where are they written?