It won't be a straight out doubling for the number of actors, as not all are linked to the trains.
Though, in saying that, the attraction can probably run with about 14-15 actors ( ) (including actors to cover breaks, to ensure the same number are in the attraction at any given time) as a bare minimum at the moment. On top of that, they would require 6-7 ride hosts too (again, as a minimum, but also including people to cover breaks). There's currently also a couple of roaming actors outside the entrance, but I imagine these are more 'new ride hype' actors, which usually disappear, as is the case for many attractions.
Last year, DBGT was running with 6-7 actors and 10-11 ride hosts. So the number of total staff isn't significantly greater than how it was last year. And it's currently less than when DBGT originally opened.
Equally, the park is a lot more Entertainments-focused now, with a much larger Ents department and budget. With events most of the year, the Thrillmakers show outside of events, the park clearly have a greater scope to ensure actor levels remain high. The issue will come, somewhat ironically, around Fright Nights time, when (as with most large horror events in the country), they'll struggle to suitably staff the FN attractions, and start to pull actors from the permanent rides.
TWD:TR hasn't had actors in any of my goes this year...whether that's bad luck or because the focus is away from that now, I'm not sure.