CK3 Title Limitations

[VIKINGR1993]

ok so i am playing as the count of rhodos, and slowly am working on becoming emperor of the byzantine empire...i am currently a duke of achaia and crete currently holding those dukedoms and the county of rhodos and working to claim the duchy of aegean islands, duchy of cyprus (creating it), kingdom of cyprus and crete (creating them), and creating the kingdom of hellas (as soon as i get the duchy of athens)..my question is can i hold the 3 kingdoms (hellas, crete, cyprus) and have no issues with vassals opinion? how many kingdoms can i hold before emperor status and before vassals hate me? how many duchies can i hold as a king before vassals hate me?

[CrUsHeR]

Kingdoms are not limited by their total amount, neither economically nor for vassal opinion. Likely you run into the vassal limit, that's why you need to become emperor at some point to increase the vassal limit, and delegate administration to other kings.

The amount of Duchies you can hold has a soft cap of 2, with every additional duchy title held giving a stacking opinion modifier of -15 from all direct vassals. Unlike your domain limit (counties+baronies) it has no direct economical impact, thus holding several duchies is something you'll be doing most of the time.
With primogeniture to make use of several duchy buildings, and with partition to have extra titles for extra heirs.