I definitely agree with what Karen said. Sitters are independent contractors and we set how many dogs we will accept and if we accept multiple families at once.
The best option for you is to contact the Rover sitter and ask them about it. There could have been a miscommunication somewhere. Or the dogs that were there could have been leaving the day you dropped off but hadn't been picked up yet and the ones there when you picked up could have been dropped off prior to your pick up. It would be unfortunate to be harboring ill feelings if there was no intent in not meeting your expectations. I probably over-communicate to owners what dogs will be with me and when but I know that some sitters rely on the information on their profile to convey if they take multiple dogs and/or their expectations (there is nothing wrong with either way).
I would also recommend for your next stay that if you use that sitter or another - even one that says they only take one on their profile, that you request your dogs be the only ones that are there. If it's a problem for you to even have overlap make sure that is clearly noted. As a sitter I consider that if you're picking up on Monday, having another dog come on Monday is not an overlap, regardless of their drop off and your pick up time. I've had clients that either have dogs that don't get along well with others or the owner just prefers that their dogs be my only clients. In the cases where I've taken the dogs it's been at a higher rate.
Regarding your comment that your dogs' care was hindered - having multiple dogs should not affect their care in any way. A good sitter can have multiple dogs and give them all the attention and care that they need. In some cases the dogs have an even better time because they have other dogs to play and interact with. If you received updates during their stay and the sitter has reviews those should be the best indicators of the care.
Good luck getting this sorted out and with your next booking!