Are there more biters in multiplayer?

There is no difference between singleplayer and multiplayer enemy spawning while using equal settings.

More Biters will appear over time while the server is running, even if 0 players are online. That's configurable (there is a setting to auto-pause the game while no-one is playing).

How many Biters spawn is determined by the evolution factor, as is stated in the wiki:

Besides choosing what kind of biter will be spawned, the evolution factor also influences the spawning interval. This interval (spawning_cooldown in the enemy-spawner definition) is interpolated between 360 (0 evolution) and 150 (1 evolution) game ticks (= 6 to 2.5 seconds)

There are 3 things that can increase evolution:

• The passage of time very slightly increases the evolution factor.
• The global pollution production increases the evolution factor.
• Destroying nests significantly increases the evolution factor.

If you want to find what evolution factor you have you can run the following command in the console:

/evolution

Keep in mind the evolution factor is in a range of 0 (lowest) to 1 (highest).

You can find more information on evolution here: https://wiki.factorio.com/Enemies#Evolution

Biter and spitter nests consume pollution, and when they consumed enough, they spawn a new enemy. So the number of biter spawns is a function of how much pollution the player(s) generate.

More players are usually more productive and thus create more pollution which in turn results in more biters getting spawned.