Choose ‘e‘ and you get to a command line where you can add options.

You can also create permanent boot option in grub by making a copy of your current boot option and add maxcpus=1 to the boot parameters. You can name this ‘Ubuntu, kernel 2.5.12-9-386 (1 cpu)‘ and have it amongst the other boot options like the picture shows.

You can verify running on 1 cpu by issuing:

cat /proc/cpuinfo   | grep processor this should show something like this:

You can dynamically disable CPUs via /sys/devices/system/cpu/cpuN/online, for example to disable CPU 1, use:

echo 0 | sudo tee /sys/devices/system/cpu/cpu1/online

cat /sys/devices/system/cpu/online  (to check online cpu)
cat /sys/devices/system/cpu/offline   (to check offline cpu)

re-enable, use:

echo 1 | sudo tee /sys/devices/system/cpu/cpu1/online