There are three types of characters: masters (noblemen), servants and visitors. Masters are situated upstairs, while servants reside downstairs. There is at least one servant in each scene.
Guests gather at the house of Sir William McCordle and his wife Sylvia McCordle. They have a daughter, Isobel, and Sir William has a daughter from his previous marriage, Mabel. She is married to Freddie Nesbitt. Guests for the weekend include Sylvia's sisters with their husbands, her aunt and their various friends.
Lavinia Meredith, married to Lieutenant- Commander Anthony Meredith
Louisa Stockbridge, married to Lord Stockbridge
Constance, Countess of Trentham - Aunt of Sylvia, Louisa and Lavinia.
Lord Rupert Standish (invited by Lady Sylvia to woo Isobel) Jeremy Blond (lord Rupert's friend)
Ivor Novello (sir William's second cousin)
Morris Weissman (a Hollywood producer and Novello's friend)
There are 14 masters in total.
Servants are divided into Gosford Park (the regular staff of the house) and visiting staff (those whose masters are Sir William's guests). There are way too many characters (even for Altman standards), and some of them are even uncredited. However, there are 14 most prominent servant characters.
Gosford Park staff include:
Mrs. Wilson, the housekeeper
Mrs. Croft, the cook
Jennings, the butler
Elsie, the head housemaid
Probert, sir William's valet
Lewis, lady Sylvia's lady's maid
George, the first footman
Arthur, a footman
Dorothy, the still room maid
Bertha, a kitchen maid
Mary Maceachran, lady Trentham's lady's maid
Robert Parks, valet to Lord Stockbridge
Barnes, Commander Anthony Meredith’s valet
Henry Denton, Morris Weissman's and Ivor Novello’s valet
There are 14 servants in total.
All masters and servants have their own subplots, of which only some are further explored in the movie. Others are hinted, but they are there. For the subplots, check out this page.
There are only two visitors: Inspector Thompson and his assistant, Constable Dexter.
We can see that there are 30 chatacters in total (31, if you count sir William's dog), not counting nameless servants - althought it does look like there's a lot more.
There's no such thing as "protagonist" or "main character" in this movie; there's an ensemble cast that suits movie purpose perfectly. Still, there are a few characters that are more prominent, or with more explored subplots. In masters group, these are sir William, Sylvia and Countess of Trentham. In servant group, these are Mary, Mrs. Wilson, Robert Parks and Elsie.