I'm not sure what you mean by the first question but
one (1) NM-HDV=, two (2) PVDM-12=, and one (1) VWIC-1MFT-T1 supports 24 channels of medium complexity VoCoders: G.729a/b, G.726, G.711, and fax
see: http://www.cisco.com/en/US/partner/products/hw/routers/ps259/products_data_sheet09186a0080194e20.html
So if you hv one NM-HDV and one VWIC-1MFT-T1 configured for 24 channels, you have three slots left for PVDM-12.
Each PVDM-12 can support three conference sessions. Each conference sessions can support six calls.
So to answer your question, yes that is 18 calls but you cannot have all 18 calls in one conference. For that you will need Cisco Conference Connection.
I'll leave the bandwidth questions to the experts but that seems right to me.
H. M.