Depends what level of discount you want and what additional perks. From their Benefits page:
Buying 2-19 shares will entitle you to 5% discount in our bars, and if you buy over 19 shares you will get 10%!
Online discount is double of that.
If you invest in several tranches, you get Reinvestment benefits, like free exclusive beer, extra discount vouchers, etc.
There are additional perks at higher investment levels, all listed here:
https://www.brewdog.com/equityforpunks/benefits