If you're new to the game, I really would just stick with 1 or 2. I only have so many because of how long I've played. I only have all of these "mains" because I put the time into them after learning all of the game's general mechanics. When I first downloaded this game, I stuck with Ganondorf for a really long time until everything was super comfy and branched out from there with the melee and 64 vets that I played back in the day, then the Brawl characters I liked, and now there's P:M specific characters. Once you git gud playing with more characters every now n' then will only help, even if you don't take them to tournaments. Knowing how to play more characters means you know more about matchups, etc.I guess, I need different play-styles to keep myself active and busy, I think I can manage four. And two of them are very similar.
If you are new to the game and trying to decide between Marth and Roy, I would pick Marth. He has all of the essential skills you need to play this game. You'll learn how to micromanage spacing, dash dance circles around your enemies, and he really pushes the envelope of killing the opponent fast or being punished because you couldn't kill them fast enough. That's a skill that strangely not too many players have. People will beat around the bush with flashy stuff when they could just kill and get it over with. In a tournament, all that matters is who has stocks left at the end of the match. You won't get more money for looking good on stream.