This parameter is documented here: https://getstream.io/docs/#following
It means the number of activities to be copied from the followed feed to the following feed.
It's limited between 0 and 1000 with default 100.
If you want to start getting updates when a follow relation is set up, you can set it to zero.
If you want to provide some historical activities, then you can set it up to 1000.
Each activity copied counts as an update on your plan.