I wouldn't presume to suggest how easy or hard something is, but the station already shows a widget for each train, so the number should be nearby somewhere.
And similarly, if the all trains GUI (top right button) could also show the total count of trains. Purely for a 'wow this is using a lot of trains' factor. I've seen a number of megabase video tours that say 'yeah we're using over X trains' but it'd be great to just open that GUI and say "yup, this base is using 2000 trains"

Thanks.
Edited to add the image.