I haven’t found a notice on the UI (Charater Sheet/Skills) how many skills are currently in the queue and what’s the maximum allowed number of queued skills.
I’d recommend putting that information in the second pane in the headline (the line with the burger menue). So it will say in the headline: “Training Queue (39/50 skills)”
I’ve never had to think about this, but can definitely see where it would come in handy, especially since the 50 skill limit is never explicitly mentioned in the in-game UI.
iirc, the only two times this limit is ever expressly mentioned in official EVE Online documentation is in the following two support pages: