This is a really good question.
Webster defines slut : a promiscuous person : someone who has many sexual partners
and
promiscuous : having or involving many sexual partners : not restricted to one sexual partner or few sexual partners
and
many : consisting of or amounting to a large but indefinite number
So the definition is a bit vague not actually quantitatively defining what 'many' is.
My late wife dated from her late teens to her late 20s, about 10 years before marrying. Let's surmise that during that time she dated a guy for only a year, and it's not unreasonable to assume that during a years dating she was intimate with each of them. That then means 10 different guys, but we all know that many relationships either end in marriage or they end in much less than a year, so for this purpose let's discussion let's say 6 months. That then means she had 20 different men. Does that then classify her as a slut?
Many men here call their wives sluts because they have had 1,2 or 3 men before them. I guess we're back to the term many. If you want your wife to be virginal then 3 is many but if you want her to be slutty 3 is a drop in the bucket, but so is 20. So, what if you define 3 different men in a 10 year dating period not many. Then she isn't in your mind a slut but what if you found out that all 3 were at the same time. That doesn't fit the classic (Webster) definition, what about your own definition is she then a slut?
If she dresses provocatively do you consider that slutty and what is slutty dressing anyway? My late wife was a conservative dresser, not provocatively. Just wasn't her but she was a slut, buy anyone's/everyone's definition.
Why do I call her a slut, She rarely talked about her sexual past but in a moment of weakness she told me that she had sex with 300+ different men, then paused for a moment and added probably closer to 500. To that add as a hotwife for just under 2 years she did 100+ different men.
