In Pokemon Sword and Shield there are a total of 8 gym leaders. They all are different types of gym leaders. First there is the gym leader Milo who is a grass type gym leader. Then there is Nessa the water typed gym leader, Kabu the fire type gym leader, Bea the fighting type gym leader, Alliseter the ghost type gym leader, Opal the fairy type gym leader, Gordie the rock type gym leader, Melony the ice type gym leader, Piers the dark type gym leader, and Raihan the dragon type gym leader.
After you have beat all the gym leaders you would have to battle the current champion Leon who have passed all the gym leaders and in order for you to complete the game you would need to beat him in a battle.
There are also ways to beat the gym leader since you know what type of Pokemon that the gym leader will use. For example since since gym leader Milo is a grass type it would be most effective to use a Bug, Fire, Flying, Ice, Poison type pokemon against him to increase and almost guarantee your chance of winning and getting the gym badge as long as you keep your Pokemon the same level as his Pokemons or even higher level then his. Electric and Grass typed Pokemons would be strong against gym leader Nessa's Pokemon. Water, Ground, and Rock types against Kabu. Fairy, Flying, and Psychic types against Bea. Dark and Ghost types against Allister. Poison and Steal types against Opal. Fighting, Grass, Ground, Steel, and Water against Gordie. Fighting, Fire, Rock, and Steel against Melony. Bug, Fairy, and Fighting against Piers. Dragon, Fairy, and Ice against Raihan.