I usually go with an aerobottle and a downtube bottle. I usually have one other bottle on my frame as well, but it is for nutrition and I don't count that bottle as part of hydration (since only a couple sips from it an hour). I find the aerobottle is about all I need, but in case the bike tips and that empties, or if the aid stations are a bit far apart, or if it is really hot, I like having that bottle on the frame as backup hydration. That works for me in races with a bottle pickup along it. At the pickup I just refill the aerobottle and continue on my way. It also avoids the problem of needing to be able to carry bottles from the pickups on my bike -- I have a small frame and they don't always fit. I do try to know ahead of time what bottles will be handed off so that if I need to I can have a plan in place for carrying one.

Depends on:

1) what you plan to eat and drink (how many calories and how are they delivered);
2) your sweat rate for the likely race temperature; and
3) number of bike aid stations during your race

As an example, my current setup is to use concentrated Infinit for distance races, with 1 900-calorie bottle per 56 miles plus an aerobottle full of water which I refill at aid stations as needed. (More when it is hot and humid, less when it is cool). I'll take 1 or 2 gels in that time to break up the monotony and maybe get a bit of caffeine and/or protein. In a HIM, that means just 1 bottle on the seat tube and 1 aerobottle full of water between the bars.

A hydortail or similar is nice to have for training but may not be necessary during a race depending on your setup and the number of aid stations. If there are plenty of aid stations and you are good at quickly doing what you need to do with the bottles, you can skip the hydortail. You can also often store an empty in a jersey pocket if you miss the drop zone.

I got by with:

1 water bottle - hydrotail (btw;what a cool word)
1 "food" bottle (1,000 calories) - hydrotail
"emergency" 8 oz. fuel belt bottle w/ water (which i did not need.)- shirt

there were aid stations every 14 miles but if it was 20 miles i would have needed more water. in retrospect this may have been a dumb plan, especially if the wind picked up.

The bottle drop is just that...drop a bottle, pick up a bottle.
Like PJT said , depends on the aid stations and what's offered and what your needs are.
I drink what's offered on the course so I get by with two, even in IM's.
If you are drink some special rocket fuel then you'll probably need more depending on how you're mixing and such.

I'm with Kylie: aero bottle plus a nutrition bottle on the downtube. You can grab-n-go at the aid stations to fill your aero bottle easily as needed. Very low maintenance.

My first IM, I had an aerobottle on my handlebars, a downtube bottle and 2 bottle on the back of my seat. There was a generous amount of bike aide stations for bottle drops.

Second IM I had my aerobottle and 2 bottle on the back of my seat for the 1st 56 miles, and then dropped down to aerobottle and 1 bottle for the second. I knew the set up for the aide stations and trained on what they were providing.

I'd measure how many ounces of fluid I have on my bike rather than how many bottles. Also, does the race have a bottle exchange aid station?

Matt Cazalas
Technical Writer
network cables

I have Infinit concentrated for 3 hours (300 calories per hour) on my down tube, and water in my aquacell.

In Cancun two weeks ago, I also kept a bottle of water on my hydrotail for, uh, toilet flushing purposes when peeing on the bike. Worked out great, and I doubt the extra bottle slowed me down much.