I have two questions:
a) Anybody know how can I calculate the overhead in VPN MPLS network ?
b) And about Voice over VPN MPLS, how much Kbps need I reserve to each voice channel ?
there is a minimum of two MPLS labels (i.e. 8 Bytes) per VPN IP packet in most cases.
To calculate the VoIP bandwidth per call you need to account for the additional 8 Bytes. F.e. Voice with G.729(a) 8 kbps voice data, 20 ms (=Bytes) per packet
PPP on the link (8 Bytes L2 overhead), 48 Bytes of overhead for MPLS and L3/4 (2 MPLS labels + IP/UDP/RTP):
76 Byte per voip packet, 50 packets per second
gives you 30.4 kbps per call.
A different L2 encapsulation results in different bandwidth requirements.