Why should there be a need to tell them? Don't tell them.
I don't even tell the girls I date in the RW how many women I've been with; also I don't ask how many guys they've slept with. It's none of my business what they did before we started dating. I like to focus on the present and what is in front of me.
I may be the outlier here, but I'm not insecure about myself that I need to know how many guys a girl I date has been with. What does it change? Your looks attracted me, but your personality kept me. That's all there is to it.