How many players realistically possible in Pandemic Board Game 2nd edition
The On the Brink expansion does allow for five player games . I believe the only rule change in On the Brink that would affect this is only shuffling two event cards per player into the deck. I'm not sure if this would affect only playing with the base game (the expansion includes many more event cards which makes this necessary), I can't remember how many event cards there were with just the base game.

The main issue you face with playing with more people is the game's built-in time limit (when you run out of city cards). I've never tried altering the rules to better accommodate more players.

I can't remember where I saw it posted. But I believe the creators of the game said that they limited the number of players to such a small number as playtesters didn't find it to be fun having to wait so long between turns. If a player traveled to a city to treat diseases on their next turn. A game with five other players meant that ten more infection cards would be played before they had a change to clear anything up.