Darksol the Painbringer |
Well...regardless of where the thread was discussed, it doesn't help to answer the question (the former of which is an issue the moderators can fix quite easily). At any rate...
The +1 ability score increase via character level comes every 4 levels (4, 8, 12, 16, and 20th levels respectively). These bonuses can be spent however the player chooses (though the player will usually resort to spending them all on their 'primary' statistic).
RAW (that is, the Rules As Worded) makes no mention or implication of there being a specific limit to a given subject's ability scores/attributes. The only 'limit' that may be implied is the maximum the character can receive due to limits placed on methods of application. Your GM would be correct in saying that currently, the player's "Maximum Score" would be 21, as RAW they cannot obtain a higher score unless they deviate from RAW, such as more (or less) than average Wealth By Level (WBL), customized items, limited (or possibly unlimited) access to certain items that boost attribute scores, etc.
However, the GM would be incorrect in saying that a given subject can only have a maximum statistic of 21, as RAW contradicts that statement when there are multiple methods (as well as PCs, NPCs, Monsters, etc. which follows the permissible RAW listed) to bypass such a measly number, given the amount of options PCs, NPCs, Monsters, and other creatures have at their disposal.
As for the reason behind the GM saying it, it could be a mis-mention, in saying that the current maximum available to your characters is 21 (as in it is limited due to lack of available/usable options) in a given statistics, or it could be a houserule, in which case you should talk to your GM as to the reasoning to uphold that houserule.