Congratulations! You are in possession of a smart creative child with a VERY active imagination. You are going to have a great time rearing this little girl. My daughter was much the same way, she was constantly intergrating reality into her fantasy world. She's thirteen now and is an excellent writer on the plus side and also a convincing liar on the down side. Perhaps she will be an attorney..haha...anyway don't worry it, it's a phase that will pass.
If she is escalating her number of imaginary friends someone in her life is probably reinforcing her fantasy world. Do you laugh when she spins her yarns or bring it up to her as if the person is real? Asking questions about the imaginary friends really feeds the childs desire to expand on the story. My Mom was so bad about that, it drove me crazy!
As for her tattling to her imaginary friends be sure to reinforce to her that you and her father are (to be blunt) the boss of her and her imaginary friends. She is just challenging your authority in a new way (because she's smart). Be sure to hold her accountable for what she does rather than letting her blame her imaginary friends, she is looking for ways to manipulate and get control. All kids do it, some throw tantrums, some whine all the time, and some try to outwit you. Children are just little people who often go underestimated. I'm not trying to insinuate that she is cunning or bad, just a little human trying to get what she wants and/or needs.
By the way, one is very young for an imaginary friend, they usually arrive around three. Your daughter is precocious, you should be very pleased. Keep on your toes and she is going to delight you throughout the years. :)