Every unit can hold up to three designations with different class and units. When selected in the team, each character has some unique abilities that can help the team in the game. But it also depends on the positioning of those units. The number of units that you use in a team while playing is not limited as you can use multiple units of the same designation at the same time. The visibility of the tiers of champions depends on your rank as well. You see the stats as per your current rank. You can only see the stats of the player that are one level below you. Once your rank and level increases, you can see the stats of players higher in the ranking process, but still below you.
Which champion should I use?
The Champion Pool is shared with every other player. You need to check regularly which champion is used by others and what specific skills he has. Once you can confirm the champion which has been used the most in winning streaks, you can decide to use it. But you also need to make a strategy as to how it will be used in the gameplay. Once it is decided, you will be able to play the character properly and to its full advantage.
Keep a check on the latest stats
The best way to stay on the top of the game in TFT most is to regularly check the stats of the champions in the game. The champion with the highest winning stats has more advantage over using a different champion. So keep a track of their winning. Also, you can use lol boost ranked to further increase the rank and skills of the champion which can give you an added advantage over other players.