How long have you been married?
How old are the two of you?
Was he seeing her before he met you or did he cheat on you with her?
Have you confronted him about the number?
Well we both know that's a bunch of hooey for him to say he doesn't know how it got in there. Either he put it there or she did. If this is someone who he has cheated on you with, then there is absolutely no reason why her number should be in his phone. This could be easily remedied by looking at his cell phone bill to see if he's called her. If he's not guilty then he should have no problem with you looking at his bill.
I welcome your thoughts, let me know if you want to talk more
I had no way of knowing that you checked the bill since you didn't say that initially. Since I don't know you or all of the particulars of what's going on, and it's not abnormal for some of these conversations to go back and forth a half a dozen times or more while we work on the problem.
As to your initial question "what would you do". It's obvious that he hasn't stopped talking to this woman and there is still some connection he feels toward her. He not only has deceived you by keeping in touch with her, has lied to you about keeping in touch with her, and tried to hide the number in his phone. How does he expect you to believe that there's nothing going on? The best indicator of future behavior is past behavior, if he's cheated and lied before, chances are he'll do it again. He's not even made the effort to cut ties with this woman who you call a crack whore, yet obviously he feels some type of bond to. You have to make a decision on whether or not you want to continue to put up with his lies and deception, or if he continues to deal with this 'crack whore', risk that he may bring some sort of trouble into your home in dealing with this woman, whether it's developing some drug habit of his own in his dealings with her, or bringing home an std. Ultimately since he isn't able to exert any self control, you need to do it for him, you can give him an ultimatum (her or me), but there's always a chance that he will challenge the ultimatums and make a choice you're not happy with. You can try and talk to him again and demand that he stop, but it seems that hasn't worked either. So you have to decide if you're willing to be disrespected and lied to, or if you've had enough and would prefer to try and find someone who will treat you like you need and should be treated.