I have the Square D cat # QOC-20M Series E6 Box but I want to add another 30 amp breaker to this. What is the part number for this add on?
Thank you
Ray

Go to Lowe's or Home Depot. Look for Square D 30 amp breaker.
I think there is two versions, Homeline and others. Get both. Use the one that fits, return the one that does not.

"What is the part number for this add on?" You asked about a replacement breaker but then closed with an "add on". Are you asking about a breaker or adding a subpanel?

QOC-20M is the cover number.
The breaker you seek is a Square D "QO" series.