The “running” field in top doesn't show the number of tasks that are simultaneously running, it shows the number of tasks that are runnable, that is, the number of tasks that are contending for CPU access.
If top could obtain all system information in a single time slice, the “running” field would be exactly the number of tasks whose status (
S column) show
R here is often said to mean “running”, but this really means “runnable” as above). In practice, the number may not match because top obtains information for each task one by one and some of the runnable tasks may have fallen asleep or vice versa by the time it finishes. (Some implementations of top may just count tasks with the status
R to compute the “running” field; then the number will be exact.)
Note that there is always a runnable task when top gather its information, namely top itself. If you see a single runnable task, it means no other process is contending for CPU time.