I would suggest selling the mackie and getting a Behringer XR18. Includes 18 usb outs you are looking for at a very reasonable price. And as Bradley says, these are not your old typical Behringer products.
This is probably the most cost effective option. Otherwise, you'll end up buying something like the XR18 anyway, unless you got a used Alesis HD24, but then you'd need preamps. However, the HD24XR has fantastic conversion, can be had fairly cheaply if you can find it and is good enough to make records. It was the basis of my mobile rig, but then there were the Grace, 7th Circle, API, and other preamps to factor in.
Another option, although you lose the ability for everyone to make their own mixes, is the Metric Halo 2882 plus a used Presonus Digimax LT. It has an 80 bit digital mixer built in with preamps, conversion, and DSP that stands head and shoulders above all the prosumer gear mentioned above. (Disclosure: I'm a beta tester and long time user of Metric Halo).
However, the Behringer is probably the most cost effective.