one song per session.. that's the way to do it.... especially if you track anything to send to on to a professional mix engineer... and even more so if it's going to come out on a record label after that mixer is done.... "every song in one session" pisses most mixers off, and the last thing you want as an engineer is the mixer telling the label that YOU did things in a way that caused them any extra work at all. believe me, if you cause the mixer any extra work in this scenario, you WILL be thrown under the bus.
this may not apply for mixers like Ballou that mix almost entirely with hardware.... those guys treat the session like a reel of tape, so it comes down to being aware of what the mixer will want.
easily 80-90% of the time though, the mixer will want individual sessions.
i know many of you will want to debate this one... but it's usually best to do separate sessions...trust me.
it bears mentioning, especially to those of you that will say "but i always mix everything i track", that you will not always have that choice, even after you've mixed. if a band gets signed after tracking and mixing with you, your mix is definitely not guaranteed to be used... and you'll be expected to deliver sessions to the mixer that's chosen for the label release.
the best bet for everyone here looking to move forward in the industry is to do things in a way that makes you easy to work with.
for the record... before anyone pisses and moans about my "attitude"... i DON"T CARE what you do. this is just advice, based on real-world experience... and it's good advice. do whatever you want though.
also, this is like the third of or fourth thread on this same topic in the last couple years. like, exactly the same topic.