Hi Giomurva,
If you are talking about actual teu (twenty feet equivalent unit, the standard for container capacity) on board then you won't find that anywhere.
If you are talking about the maximum amount of teu a ship can load then you can find that on several shipping sites. Please note that the maximum capacity is in general a much higher amount then the actual maximum amount a container ship will ever carry.
This due to unused slots (over-wide cargo etc.), line of sight forward of accommodation, high-cube containers instead of standard high containers, crane reach in ports on route, weight distribution of cargo (stability) and several other factors.
I have been sailing for many years on a 7000 teu vessel and we never carried more then 6100 teu, and that was full load.
Cheers,
Roy