How many 2.4 mile swims will you do before IM?

Maybe 2

I should have mentioned something like this with my original answer.  Interval workouts are definately the way to go if building swim fitness is your goal.

I'm doing one OWS session a week simply because it's not as boring as swimming in the pool on my own.  My other two swims are with a Master's class that keeps me accountable.  When I go to the pool on my own, I tend to drive past the pool by "accident" a few times, and when I do get there, I am always tempted to cut the workout short. 

I don't mind swimming OW as much...mainly because I actually feel like I'm going somewhere and I don't need to count laps.  Once I swim down...I have no other choice but to swim back so I'm committed to the distance.  I usually get to see some training partners at the beach I swim at as well so it's a social bonus for me.

Some others already mentioned it, but swim fitness is built up better in the pools by doing lots and lots of intervals. For getting used to swimming open water then doing open water a few times helps out. As I've become better at constructing and executing interval sets (both in masters and on my own) in the pool, the continuous open water swims have become much easier. I've needed zero practice to keep right on going and doing better than ever with it. The pacing is noticeably lower than the intervals, so it's easy to keep right on going without the mini breaks.

I try to do 4-5 and have 3 in open water. I will try and do a couple ows swim/bike brick

I taper longest on the run, medium on bike, and will swim and do very short swim taper. If I don't do late season IM, I do more ows during the taper.

For me my swim fitness effects how the rest of IM day. Longer swim time means longer time to not take in calories and fluid.

That line of thinking will get you there, but you get more out of the workouts by incorporating intervals. You can swim hard for more of the set, more than you could by going straight through, and swim with better form much deeper into the set. You do want to keep the breaks rather short though, then your ability to control form sort of resets fairly quick so that you maintain the good form and your body still sees the workout as rather close to going harder continuously. Do more work consisting of 50-200m intervals on say 5-15 sec or so rest. If you can set this up as a send-off interval, even better. Sometimes get in some work with some fast 25's and 50's to help you find your form at faster speeds even more so.

Swimming development can flatten out rather fast if you only do continuous swims all the time. And even though your goal is to just finish, I don't know of anyone who wouldn't mind a bit more speed or conditioning to make the day more enjoyable.

IM less than two weeks away, and I can now say definitely 2