Well I think that people who sleep with a lot of people and DON'T get anything are very very lucky (or just don't recognise [or get] symptoms/get checked).
I also think that if someone is single and having a bit of fun it doesn't necessarily
have to be branded as slutty unless they are harming someone with their actions.
Eg 1 single girl/guy sleeps with someone for fun, has fun. Enjoys themselves. Slutty? Perhaps yes, perhaps not, we don't know all of the details.
Eg 2 someone taken cheating on their partner (in a closed relationship). Slutty? Well they are hurting their partner... I don't know if I'd call this slutty or immoral though.
Eg 3 prostitute or someone trying to get a promotion or similar. Sleeping with people for money/items/promotions. Slutty? I think most of us would say yes. Immoral? I'd say yes.
One could argue "what if the prostitute is trying to feed her family and has no other skills?" type of argument but what stops this person from getting a normal job, time? Money? There is (cheap and even government funded) education out there or jobs which pay a bit but don't require much skill eg call centre jobs, a lot of which have uncapped commission.
Is any "number" slutty or is it how this number is achieved? Are we confusing slutty with immoral here? Is there anything really wrong with a single person having fun anyway, or are we being a bit too judgemental? I personally wouldn't sleep around for fear of STD's + I only do things (including kissing) when I am in a relationship with someone and I'm fine with not being in one at all but not everyone is like this. Some people crave the physical side of things. I don't think it's slutty unless it's in exchange for something other than fun OR if it's hurting someone ie the partner of the person they are sleeping with (if they know, though it's the person's fault more than the person sleeping with them especially in an unknowing case).