In chapter 10, as the party is leaving Southerton, Mrs Norris says, “…That Mrs. Whitaker is a treasure! She was quite shocked when I asked her whether wine was allowed at the second table, and she has turned away two housemaids for wearing white gowns.”

What the servants ate (the second table) varied from household to household. According to J. Jean Hecht in The Domestic Servant in Eighteenth-Century England, servants’ meals were usually based on what was served above stairs. She says,

“The food was usually much the same as that prepared for the master; the remains from his table constituted its staples. In the more elaborate establishments, however, the upper domestics took their meals in the steward or housekeeper’s room, at what as termed the second table, while their inferiors ate in the servants’ hall. The meals provided in such households for upper domestics, as a rule, surpassed in quality those served to the lower staff. The former were based on the cuisine of the first table; the latter were specially coooked plain fare to which were added the leftovers from the second.”
(page 109)

This book goes on, in detail, to discuss the discrepancies between the diet of upper and lower servants.

As to the cook preparing several meals, remember that it was, usually, not just the cook, in the kitchen but kitchen help and scullery maids under her direction. The number of meals prepared was likely related to the number of people available to prepare them.