Probably a bit late, but for others maybe. The subject is not well documented ... and this is for v7 (maybe also for older versions)

I just tried to allow more CPU cores to Openerp v7 and find only a few here : https://www.odoo.com/fr_FR/forum/help-1/question/is-openerp-6-0-wsgi-complaint-10657 and there : https://www.odoo.com/forum/help-1/question/difference-between-built-in-multiprocessor-mode-and-gunicorn-1671

You need to modify the config file (for debian like : /etc/openerp/openerp-server.conf) used at startup. Change "workers" value at 2*cores +1 . I had to modify "limit-time-cpu" to 3600 (1 hour) as long imports where failing with a "CPU time limit exceeded" error. I think this kind of file doesn't support comments, so I did copy the original before any modification. You need to restart the server with a "service restart openerp". That's all.

Enjoy

Updated : Also had to change "limit-time-real" to 3600 (I had undocumented errors - a thin rose line - while importing big files).