With over 1000 Pokemon and so many of them with alternate forms, there are more than enough Pokemon that can battle against others, whether that's between Trainers (and Pokemon Scarlet & Violet's Indigo Disk DLC has shown how engaging even battles against AI Trainers can be) or in the wild. There are so many that players have countless options available for team building. Even so, there remain some forms that cannot be caught in the mainline series at all, and certainly cannot be used in battle.