We have 6 sandbox systems distributed distributed over 3 servers (2 systems per server). The configuration/development team has to duke it out amongst themselves which one is used for what purpose. I (Basis) stay completely out of that discussion other than telling them that they can't get any more sandbox systems. But I also try to be very flexible and responsive when they want/need a sandbox refreshed from production. Those requests have to come from the team leader or be explicitly approved by all team members.
After a refresh, they have the option to have the sandbox configurable, but in that case the system will not receive any transports other than patches from SAP or vendors, and any transports generated in a sandbox will never be exported or imported anywhere else.
If they choose to have the sandbox not configurable, i.e. locked down like production, then they can request any transports from development they want to be imported into the sandbox.
Hope this helps.