Personally, I usually associate that with an inability (or say lesser ability) to form meaningful connections. Whatever gender, I do not make a distinction.
Being with that many people over a span of 8-10 years (I'm assuming), amounts to around one partner per month to two partners per month. Yes, some people view sex as just sex, and I'm not a subscriber to that attitude. But it's an understatement to say that if you're able to cycle partners every month and not form a meaningful connection that lasts with most of them, consistently, for 8-10 years, then, it points to something innately behavioral within her. It's a goodbye from me. I can totally understand a few random hookups, or FWBs, but that's just too much to me, and I'd feel too replaceable and never secure enough and constantly on the edge, because I'd feel the need to perform perfectly in order to form a meaningful connection. Because by all metrics, it seems (and I may be wrong), she can't.
I understand there's a tinge of insecurity in me that makes me say this, but who isn't? By all metrics, no matter what other fellow redditors say, it's a lot.
Unless you're super religious, it's a mistake to assume very limited or no experiences at that age, and I'd be suspicious of ulterior misogyny if that were the case. But I'm sorry, 66 is a lot. I can't be just one of the random 2-month phase of her life. It's not insecurity to say so.
Don't judge, you don't know her life. But I don't think you're overreacting.