NCH Swift Sound advertises that IVM can SIMULTANEOUSLY answer up to 64 lines.
While I have not performed a benchmark to measure the timing of the answer, I can tell you that from my experience, IVM appears to perform as advertised.
I am currently running IVM on 3 lines (using a Dialogic 4/PCI). I have simultaneaously dialed two of the lines at the same time and did not notice any appreciable delay. (I say appreciable because I placed one call using a land-line, and the other from a cell phone. The number I dialed using the cell phone, did answer about 500 ms after the first. However, that delay could have been created by me dialing and/or the extra call routing going through the wireless provider's network.) In any case, it was un-appreciable.