I assumed that servants are not mentioned unless part of the character development or plot building. I never wondered any of this -- it is one of the reasons I like group reads!

In other Austen novels, servants are taken to be background -- just as one imagines a floor and walls when only the furniture is described to a room, I guessed that a comfortably rich household would have housekeepers and under grooms, and gardeners, and all the things that are not mentioned in Emma's intro. Likewise, I thought Mr. Knightley walking in, meant having a servant announce him as the door was opened. But I will be interested in other musings and solutions, now that Pemberlians have brought up the topic!