Rihanna. She already has 11, Mariah has 18. We all know Rihanna likes to release new music every year and she does lots of collaborations. I wouldn't be surprised if she could get there in the next 5 years.
It depends, if Rihanna keeps churning out an album every four and have roughly four singles she could surpass Mimi. If Katy has an album that spans 1/2 years with bearly every track being a single then she could take it. They would have to still be popular and mainstream though to achieve it.
I love Lady Gaga, but Rihanna looks to be the woman to beat. I honestly think Rihanna will NOT ONLY beat Mariah Carey for #1's but also beat the Beatles record of 20 #1's. Mariah Carey is definitely NOT getting another #1 anytime soon and the woman currently in 2nd place (Madonna) doesn't look to be catching Mariah Carey anytime soon either.
Katy Perry more than likely will not continue her string of #1's. Rihanna has managed to get #1's with collaborations & having her own songs catch on like crack. I have a feeling that Rihanna (within the next couple years) will have at LEAST 9 more #1's both as collaborations & her own song. You just KNOW the lead single from her next album WILL be #1. That's a given.