The docker-compose documentation states that:

cpu_count, cpu_percent, cpu_shares, cpu_period, cpu_quota, cpus, cpuset, domainname, hostname, ipc, mac_address, mem_limit, memswap_limit, mem_swappiness, mem_reservation, oom_kill_disable, oom_score_adj, privileged, read_only, shm_size, stdin_open, tty, user, working_dir

Each of these is a single value, analogous to its docker run counterpart.

But the docker run documentation they reference only mentions cpus. There's no mention of cpu_count:

--cpus=0.000 Number of CPUs. Number is a fractional number. 0.000 means no limit.

So what does cpu_count do?