docker-compose creating multiple instances for the same image

Updated answer (Oct 2017)

As others mentioned, the docker API has changed. I'm updating my answer since it's the one most people will probably look at.

docker-compose up -d --scale app=5

Unfortunately, we cannot specify this in a docker-compose.yml file currently (as of version 3.5).

Details:
They did introduce the scale option for version 2.2 and 2.3 of docker-compose but removed it for version 3.0. Also, to use version 2.2 or 2.3 you would need to download an older version of the docker-compose tool. The current version does not support 2.2 or 2.3 (it does support 2.0 or 2.1 however). There is also a new deploy section with replicas: 5 but it's only for swarm mode.

--- Old Answer --------------------------------------

docker-compose scale app=5

See https://docs.docker.com/compose/reference/scale/

Then you only need this docker-compose file

version: '2'services:  app:    image: app

You can do it with replica as mentioned in https://docs.docker.com/compose/compose-file/#replicas

version: '3'services:  worker:    image: dockersamples/examplevotingapp_worker    networks:      - frontend      - backend    deploy:      mode: replicated      replicas: 6

Edit (from the comments): One can use docker-compose --compatibility up to make docker accept deploy section without using swarm.

The scale command is now deprecated, you should use up instead.

docker-compose up --scale app=2

more details in https://docs.docker.com/compose/reference/up