A mouthful is the maximum amount a mouth will contain. It can also mean something that is extremely hard to pronounce, or something said that has a lot of meaning. The plural is mouthfuls.
This word falls into the category of words with the suffix -ful. While this suffix means full, it is never spelled with two l’s unless it is in the adverb form (e.g., cheerfully).
Since an adverb form of mouthful does not exist, it should never be spelled with two l’s. However several mouths full of food may be talking.
Examples
“A mouthful of Pappy Van Winkle and To’ak Chocolate is an experience that is worth having, even if only once.” [Fortune]
That’s why they changed it from Tualatin Hills Nature Park Interpretive Center. “Six words is really a mouthful,” said nature center supervisor Kristin Atman. [The Oregonian]
The former five-term mayor of Louisville will become deputy assistant to the president and director of intergovernmental affairs — a real mouthful with a lot of responsibilities dealing with the second open enrollment of Obamacare, as well as economic and education issues. [Courier Journal]
A beef brisket bun was presented in a burger carton, and immediately subverted the whole concept of small-plates eating by being enormous. It had been slow cooked and was amazingly meaty for the first few mouthfuls, but after that quickly became too heavy to continue. [The Telegraph]
In the shows the mermaids (and mermen — called princes) discreetly take mouthfuls of air from the slender breathing tubes while they perform. [Boston Globe]
Parents of children with brain tumors typically can rattle off mouthfuls of medical jargon with the fluency of a veteran Mayo clinician, particularly the unwieldy names of promising pharmaceuticals and where they stand in clinical trials. [Atlanta Magazine]