It’s based on which has the higher team level and the number of characters. So if you want to know what your “main team” would be, then you’d need to log in and check. You still have a little more than 3 weeks until the merge. If you can’t log in and want to check, maybe you can send a ticket or pm a staff here.
If you want to keep all your characters, then what you can do is buy the necessary slots on your “main team” to accommodate them. Based on the guidelines, they won’t really delete characters/companions unless you can’t accommodate them since you either lack the premium slots or you’re over the allowed 30 slots.