If you do it at your 180 day mark, you can do it online or on the phone and you will get most of what you want. If you get closer than that you may have to break up your group and make ADR's for smaller numbers.
I had a terrible time last December when last minute a few people decided to join us and I needed ADR's for 10 people. For example for a lunch ADR at BOG, I had 7 different ADR numbers. It was a pain but it worked. I had to make several dummy
MDE accounts because if I tried to add another ADR at the same time it would cancel the first one made. Sometimes if you call, they are able to combine ADR's but that was not working when I tried to do it so I stayed online and made different ADR's. When I would search for an ADR for one person I could get it but it would not give me one for 2 or 3. I also kept the times within 15 minutes of one another.
For example I had an ADR for 11:40 AM for 1, another at 11:40 AM for 1, another for 11.40 for 2 people, another for 11:50 AM for 3, and then 3 more ADR's for 11:50 for 1 person each. SO it was 7 ADR numbers for 10 people.
If you do end up with multiple ADR confirmation numbers for one meal, make sure you write them all out so they can make sure they check you in for each one of them. You do not want to get a "no show" fee.
If you are doing 15 people, you may be forced to do it over the phone but you could try it online first.
It use to be that there was a special group number to call to help with larger numbers but as far as I know that is no longer done.