Using the GetTotalUnreadMessageCount() method, you can retrieve the total number of a user's unread messages in all group channels the user has joined.
SbGroupChannelTotalUnreadMessageCountParams unreadMessageCountParams = new SbGroupChannelTotalUnreadMessageCountParams();
SendbirdChat.GroupChannel.GetTotalUnreadMessageCount(unreadMessageCountParams,(inGroupChannelCount, inFeedChannelCount, inError)=>{if(inError != null)return;// Handle error.// Further actions or processing using inGroupChannelCount and inFeedChannelCount if necessary.});