Depends on how much cinnamon flavor you want.
Adding to the whole 5 gallons when you have not done it before is less desirable than if you had divided your batch into smaller amounts (say, 4 liter jugs, smaller carboys, etc) and then added the cinnamon to just some of the batch.
In general, I'd suggest adding about 3 to 4 inches of stick per gallon to begin with ... and soak the sticks prior to adding, in enough metabisulfite solution to barely cover them for about 20 minutes (personally I'd just add both the sticks and the solution then to the cider).
The amount of cinnamon flavor is at least somewhat dependent on that "crop" of cinnamon (this is, afterall a plant product), the age of the cinnamon (how long it's been sitting around) and the thickness of the sticks.
In general, you can always add more if the initial dose does not create the amount of flavor you want ... whereas it is a big pain to try and dilute it out when you have added too much flavor.
Add the cinnamon, let it sit a few weeks and taste ... adjust ... etc