A 3ph 400amp service using 250 al can you use one conduit or two conduits, 8 wires

Not sure but I think each feeder has to be in it's own conduit.

Either.
One reason to use separate conduits is to get around the higher derating factor.

I must say, someone doing this installation should not be asking this question on a "Home & Garden" DIY message board. I hope you are just asking for curiosity or education purposes, or that you have someone on the job with more experience than you.

Also, if this is a wye 3-phase system 250MCM AL is not good for your service. You'll have 4 current carrying conductors in each conduit so you have to derate to 80%.
You'll need 350's.

See NEC 310.15(B)(5)