It's a simple matter of mathematics. Some people care about their partner's 'number' and some people don't. The majority of people who care about the number presumably would prefer it to be low, although I'm sure the slut-shamers of the world would be shocked to find that some might prefer the number to be higher. So in the in the end, if you have a high number, there are less people who consider you girlfriend/wife material. Individuals who participate in cultural slut shaming will use this simple math to push the responsibility onto others. You see, *they* aren't the ones judging other people based on numbers, instead they are communicating a simple reality about the world in which we live. They advise women not to sleep around too much because it will make it harder to find a boyfriend. You see, it's the shallow men of the world who care, they are just the helpful messenger.
But this math is deceptive. When it comes to dating pools we don't care about every heterosexual person of the opposite gender. However, it feels a little to easy to simply say "you don't want to date a guy who cares about your number, anyway." While that's certainly true, it suggests a certain moral judgment of that guy. Whether or not such a judgment is accurate, it isn't necessary to judge such men as 'unworthy,' it's enough to label those men as 'incompatible.'
All of our choices shape our potential dating pool under the above math. We limit the number of people who will choose us as mates. Voting for Obama limits the number of people who want us as significant others. But, we don't generally care. Anyone who doesn't want to date me because I voted for Obama isn't someone I want to date. Eliminating them from my potential dating pool is helpful, not harmful. Likewise for a self assured slut. If a girl has made decisions that she is ok with in her own life, then she is far less likely to be compatible with someone who refuses to date her because of those decisions. Eliminating those guys from the dating pool has no effect on finding a boyfriend or husband at all. The only people eliminated were never real candidates.
This requires several assumptions. First, it requires that women have a certain agency in their dating. In that they actually have preferences to find someone with whom they are compatible. The typical slut-shamers of the world seem insistent that a single woman should want nothing more than a male body. That she should bend her personality to his demands.
More importantly this requires healthy sexual agency among the women in the culture. As much as I don't think a sexual number is at all important to someone's date-ability, it is almost certainly important that that person be comfortable with their number. This means being comfortable with past sexual decisions. The best way to do this is to be comfortable with your sexual agency as you are making sexual decisions. This is where the disingenuous nature of those who would slut shame really comes out. If they cared about dating compatibility they would encourage knowledgeable and empowered sexual choices, instead of shoving sex into the dark.
Ultimately it's true that being a slut will limit your dating pool. But much like voting for Obama, any limitation is probably a benefit to finding a truly compatible life partner.