X-Lab Sonic WIng 140.6?

I see a lot of people at IM races using a single bottle in back and a stealth pocket type storage, but they also have a between-the-bars hydration setup.  WTC IMs are very well supported, so if your plan is to use on-course hydration (usually Gatorade and of course water) then two bottles could be enough. 

But if you need to bring your own bottles and swap at special needs then yes you need two.

I've been racing IMs since 2006 and I've completed eleven.  When I bought my P3C I put an XLab setup nearly identical to the Sonic 1406.  Two cages, CO2 inflators and cartridges, and a bag strapped in place.  I also had a cage on the downtube. This worked great for several years and eight Ironman races. I also used my own special CarboPro / Gatorade mix "fuel bottles", so started the race with two in the back, and water on the downtube.

But over the years, the CO2 inflator rusted, being out in the Seattle rain. I found that out the hard way one day.  And a cartridge worked lose one day, and fell off.  So I just put those in stealth pocket type of thing.

And a few years and three IMs back, I started using Speedfil between-the-aerobars hydration, and I really like it because it's both my Garmin mount and my water bottle. https://www.amazon.com/Speedfil-A2-Z4-Bundle/dp/B00IOY7YWQ

And I stopped using "fuel bottles" and rely solely on gels now.  I just use on-course hydration and thus don't need to swap bottles at special needs.  I just get my Pop Tarts at special needs, and more gels. I train using the same things they provide on the course, and it's usually Gatorade and GU gels. 

In my last few races, I take the downtube cage off and just use two in back and the downtube. They are easy for me to reach, never launch bottles, and I can put the bottles back fairly easily.  I think the Delta 400 type single would be hard for me to reach.