The chances of catching an STD from 1 or an 100 girls is equal.
Why?
You could be on Holiday, meet a girl, get it on, condom breaks - bam STD.
You could sleep with a 100 girls, girls that dont appear to be complete sluts and catch nothing.
Personnally, i've fuc*ed more woman then I can count, well over 100 on reflection (thats not alot in the shape of things, 52 weeks in a year, I've lived over 34 years).
I've never ever caught anything at all, I also have friends who have slept with just as many woman if not more, they have caught nothing bar thrush (just means you cant wee that well for a couple of days).
All this STD stuff, yeh its out there, but who's to say you wont get an STD from fuc*ing one woman? Nothing at all.
Yes your exposing yourself when you increase the numbers, but the odds of catching an STD per girl is about equal.
Alot of guys use this excuse to justify not sleeping with girls or plate-spinning girls, i.e "I don't want to catch something" - but in all honesty, you probably won't catch shi*.
The guys that seem to catch STD's are in-fact the guys who don't get sex, the type of guy who would fcuk anything due to his low sense of self-esteem, a guy who would simply jump into the sack with anyone due to being 'lucky' - PUA, players e.t.c usually fu** woman with standards rather then the local bike or the town tramp - those types of girls would fuc* anyone game or not - hence when a guy as developed options, I believe your chance at getting an STD is reduced and this is the only way to limit your exposure to an STD (bar being a monk).
Not by the number of women you sleep with (since you only need to sleep with one) - but by making sure the woman you choose to sleep with seem to be girls who won't jump into the sack with just anyone.
I wouldn't reccommend a guy sleep with or date the local who*e anyway - If a girl has slept with more then 10 guys, she should be ruled out (not that you can ask, but you can gauge her reaction to this rather easily, by telling her about similier situations and seeing what she thinks about it - for example "Did you hear about Jackie, I heard she as slept with about 15 guys").