I have a campaign closing in on 4 years now. Two players. System is a homebrew E6-style system loosely based on various editions of D&D.
I think each of them currently has about... 8 characters? Let me count through them
Original pair, mercenary captain (leader, support, extremely durable warrior) and his right hand man (rogue, alchemist, very mobile and high damage with a thousand tricks up his sleeve)
Second pair of PCs along with a character I made when one of them took over DMing for a six session diversion. A mobile shield fighter, a medico, and a magic-dabbling priest.
Third pair of characters, a dour sharpshooting longbowman and a dumb-as-stump axewoman.
One of them introduced a fourth character, a reedy steward/accountant/engineer for their mercenary company. The other introduced a clubfooted adolescent who ran away from home to join their mercenary company.
Then they played as a general and his second in command for a different army. Rivals but not enemies, they have worked together a few times.
Then came outright villains, working to bring down their PCs. I eventually confiscated one to turn him into a future BBEG (at the player's suggestion) and he rolled up a new villain.
Then one of them ran a few sessions for me and the other guy, so I added a second PC in the form of an impetuous noble, and the other guy added a gruff jerk who eventually died and was replaced by a combat-worthless con man.
Then the other guy brought in a Joan of arc style delusional religious warrior. And an assassin with a peculiar moral code.
I think that's it.
So I guess one of them has 7, the other has 8, and technically I have 2 who are generally NPCs except when one of the guys wants to run a mini-arc.
I'd say it's the most successful campaign I have ever run. We average 1 session per week, typically 6-10 hours long. Their primary PCs are effectively 26th level (kind of) at this point.
It's not uncommon for them to have extended conversations and even arguments with themselves. I can just sit back and watch, which is pretty amusing.
Definitely recommend giving it a whirl!