Currently Viber conference call is limited up to 20 people in a group call. Initially Viber allowed only up to five in a group call, then increased to ten, and recently increased to twenty. If you use a older version of Viber, you have to get an update to Viber App to make use of this increased group call limit. However only group voice calls can be made using Viber. Video group calls are currently not supported on Viber and video group call is limited to only 1:1 call.