24 units fully supplied according to white route, but out of supply for over a year. Did my allies take my supplies ?
Then get overrun by enemies 1/4 the size who happen to land at a naval base and are supplied. That's broke.
Why can't convoy numbers be increased on specific routes.
Without more information I am just guessing, but since you said your 24 units have a supply route, but are out of supply, it does sound like the supply route is not providing enough supply for all the units. Checking your supply map, F4 in my games, would give you a quick confirmation of how much supply is coming in and how much is needed. Unfortunately, your allies will sometimes overwhelm front lines and suck up all the supplies for themselves. It is a known issue that this forum has reported to the developers many times. It is hoped that the new DLC, Barbarossa, will address logistics, including that issue.
When the enemy landed and took the port, those units were getting their supplies from that port and removing that supply source from you, making your supply situation even worse. What is more, those enemy units invading probably had a supply grace period that allowed them to attack your starving troops with freshly fed troops.