How many UART can I use in the same time on PIC32MX795F512L?
This pin sharing is quite common on a lot of microcontrollers. It just means you can't use hardware flow control on all UARTS. I've never used that particular part but after a quick look at the datasheet it looks like you should be able to use UART 1-6 without hardware flow control, but only UART 1-3 if you need hardware flow control.

If your application does need hardware flow control you could also implement it in software pretty easily. In your receive interrupt for example lower RTS when the receiver buffer is say 75% full, and in your transmit routine wait for CTS to be high before sending.