I personally would like to have 4 maps to vote from in the lobby. Especially since we have DOme in pool, and i want to play it much much more. Not only because it is a new map, but also it is very good map.
Even with 5-maps pool i caught myself that i tend to wait for some specific maps, which are my favorite (Trainyard, Chapel) and get very sad, and even leave the server if i get 3 maps that i want to play less.