Really depends on how much beer you want to make, how long of a boil you want to have, how vigorous a boil, and so on.
I recommend a 20 gallon boil kettle to make 10 gallons of finished product because of all the losses along the way. I try and end up with 11.5 to 12 gallons into the fermenters and then 10 gallons into the kegs.
I often make hoppy beers where a lot of wort is soaked up in the boil, my boil is vigorous and sometimes 90 mins which means I need to start with 14.9 gallons (give or take). Boiling 14.9 gallons in a 15 gallon container just can't work.
Kal