With that definition mind: 4 (two and a half year twice, three years, two months).
It’d be somewhat more complicated to count all flings and one-night stands. But not impossible.
I’m kinda surprised that I’m near the top of the distribution with 7 girlfriends before I met my wife. I didn’t meet my wife until I was 34, but it’s not terribly uncommon to not meet one’s spouse until one’s late 20s or later, which gives plenty of time for a good number of relationships before then.
Two, which seems to be the largest so far. One was excusive for a year. The second was exclusive on my part, but not hers. I knew about the other guy she was dating, who was a friend of mine actually. I am not sure if he knew about me. After about 8 months, it got to be too torrid for her and she broke with both of us. It was traumatic. Her best friend then hit on me, but I wasn’t interested and the best friend later married the other guy. About 6 years later, met my wife, we were married after about 6 1/2 months and, over 50 years later, I have never looked back.
I would think from the word “exclusive” that 0 means married your first monogamous/faithful partner.