It's pretty hard to put ~1000 players in a modern multiplayer game without lag, just look at PlanetSide 2 which supports ~2000 players per server/continent - you need an overclocked top of the line CPU to achieve ~40 FPS. The player limit problem lies with physics calculations. The client needs to track and calculate the movements and projectiles of each visible player, which put a heavy strain on the CPU.
Apart from exploiting each next-gen consoles unique traits, they will take advantage of the consoles power to have much more players in multiplayer for sure - it will be a different but exciting experience to have so many players (zergs). Rockstar haven't talked about multiplayer yet, maybe that's what will be coming with next weeks second round of Previews. I think that multiplayer mode is going be big, since next-gen is made to be social and connected.