at a sitting
During one period of time devoted to a particular task. How much work can you get done at a sitting? How many people can the restaurant accommodate at a sitting?
at a sitting
at one time; during one period. (Usually refers to an activity that takes place while a person is seated.) The restaurant could feed only sixty people at a sitting. I can read about three hundred pages at a sitting.
at a sitting
At one time, during one period. For example, The cruise ship could feed about 500 passengers at a sitting, or We read the entire poem at a sitting. Since the word sitting implies just that posture, the term means "during a period when one is seated and engaged in a single continuous activity."