There is no maximum amount of Hearts you can earn on a date, but one die cannot count toward multiple stats. Once you "match" it to one of your stats, it's used and cannot count towards another stat.
I’m a bit confused, wouldn’t that still mean the max is 7?
Like here’s my math: You bring your date to there favorite location, that’s two hearts
Then, with your activity die at 6, you roll 6 dice and then miraculously they all match your six stats (That’s one for education, one for athletics, one for personality, one for style, and one for wealth and ine for creativity). This gives you another three hearts
Then (since you used all your stats) you match with your partners parameters. That’s an extra two hearts for a total of seven.
The only other thing I can think of is when you go on a special mini game date and you can gain more hearts but that’s about it.
Sorry; I misunderstood your question. I did not bother to math out what the theoretical maximum Hearts you could gain in one date is; I assumed you were asking if there was a rule that capped how many you could get (there isn't). If you have done the math and figured out that the maximum is 7, then I guess its 7! (Although I think you aren't accounting for favourite date locations.)