There are a number of issues driving where/when/how the navy can make commitments:
1. Force disposition:
East coast = 2 x Iroquois class, 7 x Halifax class, 1 x AOR, 3 x Victoria class, 6 x MCDV. Total = 19
West coast = 1 x Iroquois class, 5 x Halifax class, 1 x AOR, 1 x Victoria class, 6 x MCDV. Total = 14
2. Allied commitments:
East coast = 1 ship always committed to NATO's Standing Naval Maritime Group. Hence the extra frigates as this duty rotates 2 - 3 times a year. Every 6 years or so, Canada must provide the flagship (and staff) for this force, hence the extra destroyer. Rotating commitment to provide a frigate to a carrier battle group.
West coast = Rotating commitment to provide a frigate to a carrier battle group.
3. National commitments:
Both coasts maintain a ready duty ship to support things such as search and rescue, they also provide ships for fisheries patrols. The east coast also provides ships for northern sovereignty patrols (this summer/fall a frigate and 2 MCDV's)
4. Equipment readiness. Warships are expensive and time consuming to maintain. At any one time there is always 1 ship (or more) in some sort of work period to maintain/replace/upgrade equipment. Also, in the near future one east coast frigate will be modified to support the new Maritime Helicopter Project (MHP) and will work with the contractors to train air and support crews. In the not too distant future, the Frigate Life Extension Project (FELIX) will get underway. This will significanly extend the life span of these ships while increasing their capabilities, but will leave the navy short of hulls while underway.
As always, if the navy was told to go, the resouces would be made available. But also, as always, something else would suffer.