Here's section 2.5.5. verbatim from the EASA Approved RFM for an AS350 B3 Arriel 2B1 (a.k.a. B3+ here in Switzerland).
"2.5.5 MANDATORY EQUIPMENT
A minimum of two adequate radio / audio headsets shall be on-board the helicopter, one worn by the pilot at the controls to monitor the audio warning delivered through the ICS system and a spare one."
I don't have a B3 RFM at hand right now but seeing as the audio warning system and ICS are the same as the B3+ I can imagine the minimum of two headsets would be as well.