[CentOS] xen - 4 concurrent virtual DomU limitation - how to increae this? (centos 5 x86_64)

Is there a switch or config setting somewhere that would allow me to
start up more than the default 4 xen DomU instances?

I am finding that the CentOS 5 (x86_64) installation only allows me to
start up 4 virtual machines at a time. (It is definetly not a lack of
cpu resources, memory or disk space.)

Any help or pointers, would be much appreciated.

Regards,

-- 
Urs Rau