You currently can add as many email addresses for forwarding in "Preferences" --> "Mail" --> "Forward" option "Email addresses" as you like.
But email systems usually have a restriction, how many email addresses are allowed in SIEVE forwarding rules.
If an user adds more addresses than are allowed, SOGo will not warn him.
But the forward will not work as expected.
Therefore it would be sensible to add an Option in sogo.conf, which contains the maximum number of addresses allowed in forwarding.
With that, SOGo could reject every try to add more than the max allowed number with an error message.