According to Episode 2: Murdoch tells Annie Desmond that First Class is booked solid (which it wasn't in real life), the number for full First Class Passengers would have been 833. The Titanic had 324 in First Class when she left Queenstown.
Once the Titanic mini-series gallery is completed with First Class Passengers and servants, we can see if the numbers add up to 324 or 833.
Not all of them played people on the Titanic; you had the protestors and workers fitting out the Titanic in Episode 2, Scots guards in Episode 3, policemen and prostitutes in Episode 1, and the people in Southampton passing by during the boarding scenes.
I added a new category = Servants between 1st and 2nd Class.
BTW = for Titanic's band (2nd Class Passengers), two of them were extras and I have their names: Péter Mező as violinist with mustache and Bertalan Veér (cousin to Csongor Veér - Wallace Hartley) as older violinist.