Hi Kelly,
I don’t have evidence that would support a specific quantity, and I have found typically stakeholders rely on their own internal data for that determination. However, I have seen an approach that could be considered in lieu of permanently locating restraint beds in a space.
One organization I worked with designed a custom polycarbonate restraint board that fits between the platform bed and the mattress, with restraint hooks positioned just outside the mattress sides. This allows staff to deploy it only when needed while avoiding the permanent ligature risk and the intimidating appearance of fixed restraint loops on some beds. They would need a storage location for the boards, and would need to determine the necessary quantity. Most instances I have seen they only need one, if any.