In Shakespeare’s day the word “lusty” could either mean arrogant, or it’s current sense of sexual arousal. The nurse means the former but it sounds as if she’s saying that even if Mercutio were more aroused than he is, she would get rid of his erection, and those of 20 of his friends; and if she can’t, she’ll find some girls who can – it seems like she’s getting back into the pimping business!