Actually, that's a transformer that will provide 50kVA with a certain temperature rise and a certain expected lifetime for that temperature rise. The transformer will transform more than 50kVA with a greater temperature rise and possibly an associated reduction in expected lifetime..
Just tell me this-
You have a 50kVA service transformer. That 50kVA available (which I am thinking of as "your service"), at 100% efficiency and rated capacity, is 208.3A at 100% 240V loads and also 416.6A at 100% 120V loads.
Could be 400A, could be 600A, could be more, depends on POCO practice. It will definitely be more than 200A, as the POCO does not provide full sized transformers for the nominal service size..
So what amperage is that service? As in, when you ask the POCO "what amperage is my service?", what will they say?