Generally, to get a 350 degree inside-oven temperature the number of coals would be 2 times the Dutch oven diameter. e.g. 12″ oven = 24 briquettes. (use a little judgment here, sometimes more – sometimes less).
One charcoal briquette, or equally sized hot coal, will equate to approximately 10-15 degrees of heat on a fair camping day, (not real cold, rainy, or windy).
Generally speaking – the 12″ 8 qt. model is the standard size camp Dutch oven most outdoor campers use.
There are many variations of the Dutch Oven sizes. For example: you can get a 12″ Dutch oven that can be as shallow as 3.5″ or as deep as 7.5″.