It is a much safer option to use a transistor and a separate power source.
5v relays can not really handle much of a current.
And there are limits of how much current you can take from (source) or send to (sink) a single pin (0.04A). Load on all the pins should not exceed 0.175A (0.2A through the chip - 0.025A used by the chip itself).
Bottom lime is: if your arduino is to fail you will not only have to replace it but rebuild your project anyway.
How many (channels) relays can be used at a time? As many as there are pins available on the board (if not bother to multiplex).
Consider also using latching relays they are generally 4x more expensive that eliminates the need to keep them powered all the time.
Other thing to look at are transistor arrays - that is if you want to use lots of relays at once. That saves cost and space on the board.
Take care with mains voltage…