Is there historical data to determine the agency resources required during the contract period?
With no defined scope of work you will rely on historical data to set your agency resources in a retainer.
If the scope of the work is similar to the previous agency then if is reasonable to assume that the resources would be similar if they delivered a satisfactory level of service.
Without any historical data the setting of the agency resource level in the retainer will be a best guess and difficult to quantify or qualify.