How many sub interfaces can be created on a cisco router to route between vlans on a connected switch?
If you are alluding to a router on a stick scenario (?) then it will depend on how many IDBs the platform can handle. A SWIDB represents a logical sub-interface and you can use the IOS command 'show idb' for existing allocations and the memory consumed.
Each interface uses an IDB and the limit therefore indicates the maximum number of interfaces a router can handle.
You need to know how many IDBs your particular platorm can configure (see above). Commonly this can correlate to the max number of vlans configurable because each vlan will require 1 IDB (normally 4094 in cisco IOS).