I take it you're referring to the Info button on these respective buildings? Both have capacity information listed and in the same spot on the UI: for the Mercenary Camp, it's "Mercenary Army Capacity" and for the Barracks it's "Total Troop Capacity". Underneath each is a visual image of the type of unit and its number count beneath that. Relatedy though, what I would like to see is a 'Click on unit for more information' or comparable description here as it's not intuitive (enough) to me that you can click on the image of these units to have their individual names, strengths etc. displayed.