The Federal migratory bird hunting regulations, as well as Provincial hunting reg.s if applicable, will limit you to 3 shells.
While hunting (different jurisdictions have different interpretations, so in your area they might assume you're hunting any time you're in the woods). That's not 3 in the magazine, because at that point you could have another chambered, and thus 4 total. Your magazine must be blocked in a manner that can't quickly be removed, to a capacity of 2 of the shells you have with you. If you're being checked by the wardens and your hunting partner has short enough shells in
his pocket that 3 will fit in
your tube even though you're using super-duper magnums, they'll be suspicious of that, too. No ‘ghost loading’ or other tricks; they will rack your gun and if more than 3 come out it's being confiscated.
For semi-automatic shotguns, the Cartridge Magazine Control Regulations apply (
SOR 98-462 Part 4 S.3(1)(a)(ii)), and the limit is 5. What is not stated in the Law is that the limit is for the longest shells you can use in your gun; often tube mag.s holding 5 3,5" shells will hold 6 or 7 shorter ones. This has been confirmed by the RCMP, though it's not included in their
Bulletin 72. You can also ghost load another on the lifter if your gun will function like this, so with one chambered we're up to 9 in some guns! The only thing the Law cares about is the length of the tube.
Shotguns with a manual action have no prescribed limits. In effect this means pumps, because most bolt action shotguns only have 2-round magazines, and the break actions only hold one per barrel anyway. Functionally your only limit is the length of your barrel, because the mag. will start losing pieces if it's longer than that. This can be complicated by the fact that some manufacturers use the same tubes on pump and semi guns, so a long tube might be Prohibited if it can also attach to a semi and make it hold more than 5 as above.