In the Clan Stronghold, on the left side of the game (left of the pond/waterfall) you can see 2 soldiers working and fighting. That's your resource storage to be upgraded. A note - I just sent 14 convoys with 350k each at the same time. I have the number of marches in Invaders and Training done, and my Shrine of Odin is at level 28.

The amount you can send also depends on the level of your stronghold.