The three versions I've seen all had 10 cards of each color.
Frankly, I'm not sure that 5 cards would be very fun. It can easily lend itself to impossible games. What if no player had a 1 card in the opening hand? You can't discard or play anything to make room. All you can do is maximize your score by telling people which cards are 5s so they can be discarded. And even if a 1 is played, what if the corresponding 2 isn't out there? And so on.
Granted, with 5 players, this is a little more manageable. It's highly unlikely that nobody would have a 1. I guess if that happens, the second player will know instantly that they can't win when he sees no 1s and the first player doesn't tell him he has any 1s.
Having one of each card just doesn't sound that fun to me, but I admittedly have not tried it, so I can't dismiss it entirely. At the very least, you'd be missing the strategic component of blind discards. I like that because you can really see the anguish on the person's face as he contemplates discarding that card nobody told him anything about.
