You can have up to 3 guns per battery, and each battery takes up one Heavy FOC slot. So, if you want, you can have up to 9 guns in total, each with a crew of two (I'm not sure about more, codex is at home) and you can also take extra ammo runts to re-roll hits, up to three per battery I think.
You could take 5 of these things for the cost of one Boomwagon. They work very well in the right circumstances. And yes, as Shadows pointed out, artillery is now T7. T7 grot krew is pretty funny.
Unfortunately, here in Canada, each single, Finecast gun now costs $36.00 (plus tax)!!! And they each come with just one barrel option. I just made six new kannon out of plasticard, took me two eveningss to build and paint them, and cost me $13.00 for a bag of 12 wheels.
I thought about Zzap guns, but their random Str combined with Orks' already abysmal BS was just too much for me to bear. Maybe one day, just for fun.