It will depend on how many staff members you have. If you have 1 secretary and he/she performs a certain function repeatedly, such as taking vitals or adding patients, then another device will be needed besides the one that you use. The data he/she adds will also show up in real-time on your device.
Current SeriousMD users use different combinations that work for their needs. If your staff needs to be mobile and moving around the clinic, then having an iPad each will increase efficiency while making it look professional in front of your patients.