The problem of staffing a backup site is multi-dimensional. One aspect of the problem is determining the staffing required to run the backup data center for as long as necessary. While a skeleton crew may be able to keep things going for a short period of time, as the disaster drags on more people will be required to maintain the effort needed to run under the extraordinary circumstances surrounding a disaster.
This includes ensuring that personnel have sufficient time off to unwind and possibly travel back to their homes. If the disaster was wide-ranging enough to affect peoples' homes and families, additional time must be allotted to allow them to manage their own disaster recovery. Temporary lodging near the backup site is necessary, along with the transportation required to get people to and from the backup site and their lodgings.
Often a disaster recovery plan includes on-site representative staff from all parts of the organization's user community. This depends on the ability of your organization to operate with a remote data center. If user representatives must work at the backup site, similar accommodations must be made available for them, as well.