”Nincada -> (Level 20, empty spot in party, spare Pokéball in bag.) -> Shedinja” -Pokebase
There is not much to be said here. You need a spare spot in your party, and an empty POKÉBALL. Make sure it is a normal Pokéball, because Shedinja won’t appear in any other kind of Pokéball.
And yeah, you can have as many normal Pokeballs in your bag, as long as you have a spare spot in your party, it will evolve.
Hope I helped!