In Siege, there are 20 operators total (10 attackers and 10 defenders) with unique abilities. They're all locked behind Renown, a currency earned through playing the game and completing daily challenges. It may seem like a move to arbitrarily insert some kind of progression and reward system into the game, and that may be the case, but it rarely takes more than an hour or so to unlock an operator in late game.
Take advantage of getting to know a few at a time. There's plenty of nuance to each character, and having too many options at once might spread your skill acquisition a bit too thin. I'd rather have someone on my team that knows how to play Pulse (he wields a heartbeat sensor) super well, rather than have someone hop in and out of every operator on a whim. It may be that during the character selection screen, your only operators get picked, and you're stuck as the generic Recruit character. Don’t fret. They come with as much barbed wire, breach charges, or grenades as the other operators. They’re far from useless, and it's likely there will be enough specialized operators on your team to spearhead some kind of specific defensive or offensive strategy.