That probably depends a bit on your deployment. You will need at least one VoMP gateway somewhere, obviously.
If you have a number of MPs and you want the Serval phones to hang off those as Wi-Fi clients, then you probably need servald running on all of those MPs, so that they can route the VoMP traffic to the VoMP:SIP gateway. This shouldn't be too hard to setup. It also isn't much harder to have the servald integrated with Asterix on the MP so that each MP that will have serval phones hanging off it is a VoMP:SIP gateway. This approach would also mean that you could try it on one MP, and then roll it out to others as you have need.
A side benefit of doing this via any of these means is that you get the SMS-like functionality of Serval, so that you aren't restricted to just phone calls. Your use-case will dictate whether that is important or not.
Paul.