How to run parallel make with debuild?

It has to be enabled in debian/rules. If the package uses dh, there is a line like this in debian/rules:

dh $@

Change that to

dh $@ --parallel

Then your commands will work, at least DEB_BUILD_OPTIONS="parallel=4"

With debhelper 10, you no longer need to supply the --parallel option in debian/rules; it now runs parallel builds by default. See the release notes

The answer, is therefore, just to set the contents of debian/compat to 10 and to update the debhelper version to >=10 in debian/control.

I recommend using the DEB_BUILD_OPTIONS environment variable, as described in section 4.9.1 of the Debian Policy Manual.

DEB_BUILD_OPTIONS='parallel=4' debuild -i -us -uc -b