It is really higher.
I don't think that anyone has ever done the sort of studies/polls that could give you a percentage of risk. I don't believe it is the majority. But the problem is common enough that most rescues, for example, will not adopt a bitch out to a home that already has one... Some breeders will not allow two bitch puppies to go to the same home either.
A lot of it can come down to management too - how well you fill the alpha role, how often if at all your bitches are alone together, and how good you are at reading early posturing signs and putting an end to it at that stage. But you've obviously read some of the previous threads on this site about bitch fighting. If it starts, it is an extremely difficult thing to stop. Versus dog fighting which can be solved relatively easily. That, as much as frequency, is why it's an issue that gets stressed so much.
Anyway. Trying to work out what the level of risk is isn't really going to do you all that much good. It is by no means a certainty that your girls are going to end up fighting. But, you have to know that it's a fairly high risk. Use the knowledge to your advantage and be sure to run your household in a way that is most likely to avoid issues occuring in the first place. Be sure to take your girls to training classes, and to provide them with strong leadership that will help to deter them from fighting for supremacy in the first place. A NILIF regime is always a good one in any multi-dog household (plenty more about that on the forums). And *if* issues start to occur further down the track, don't delay in getting a qualified behaviourist in to help you deal with it.