First of all, many firearms do not have magazines. There are all kinds of firearms used for hunting and sporting activities that are a single shot, or in the case of shotguns, a single or double barrel. There's no magazine, per se, on a wide variety of firearms.
For those firearms that are equipped with magazines, the magazine size historically for hunting firearms, of the type that are widely used in Canada, would be somewhere in the neighbourhood of three or four cartridges, although larger mags are available.
For military firearms, the sky's the limit for magazine size. They can make them as big as they want and as big as is practical. The military also makes use of belted ammunition, which essentially has an infinite length.