Yesterday I have participate in non veteran campaign, with medium population in my faction active ppl was ca 15-20 players, so what is the current cap for campaign?
The population cap used to be 500 players per faction in each campaign. They reduced that to around 400 per faction. Perhaps it has been reduced even further, but it's still definitely in the several hundreds.
If the campaign was showing as "full" for your faction, there were definitely hundreds of players online at that time. If you only saw 20 active people, it only means that those were the only people in your vicinity and/or the only people bothering with zone chat. People who enter Cyrodiil in a coordinated group with TeamSpeak mostly end up doing stuff more or less without contact with others.
I don't know why it's so hard for them to just display x/max instead of the 3bar-lock system. It would end all speculation, especially since LOCK DOESN"T ACTUALLY MEAN POP IS LOCKED!