$ behave -d | grep "scenarios passed" | cut -d, -f4 | sed -e 's/^[[:space:]]*//' | sed 's/untested/scenarios/g'
-d stands for dry-run, so behave invokes formatters without executing the steps.
Then we grep for the summary line containing number of all scenarios
then we cut the last value from selected summary line that show how many scenarios were "untested" (in this context it means not executed, which is exactly what we need)
Trim leading space
Lastly simple sed to replace untested with scenarios