How can i return the max of a sum by each team?
If am not wrong this is what you need

select team,max(Total)
from
(
SELECT team, 
       Sum (total) AS Total 
FROM   campaign 
WHERE  Year(time) = Year(Now()) 
       AND Month(time) = 12 
GROUP  BY campaign_identi, 
          team 
) A
Group by team