You need to obtain 50 items, through shop, volta pass or through objectives.
In terms of Volta coins, you need 1,250,000 to buy 50 items, however ultimately you won't need to buy all 50 as the volta pass will grant you a number of items as you progress it (there are 10 in this seasons).
There are no Volta coin farming methods/glitches currently in the game as far as I am aware so you most likely won't be able to obtain this until the next season starts, which is 18 days away from the date of posting.
Alternatively, you could spend 2,500 FC points to buy 50 items if you desperately want to unlock it now, however the obvious downside is that costs real money.