Can’t say as I remember the exact number of players for vanilla servers, but I think it’s likely they will be larger than they were, but I doubt this will have a significant negative/positive effect beyond the initial rush on launch.
What you’re asking is pretty general - it’s all context really. A bigger community and server can mean a lot of positives, you are likely to have a bigger AH with larger array of items available, more guilds for a wider variety of players and play-styles, and a more active game world. It will mean there will be more farmers, but it may also mean there’s a bigger market for said materials.
I’m not sure Blizzard has announced anything concrete about the number of realms etc., that is something they will need to look into closer to launch based on metrics they have available regarding interest in WoW Classic.