Could a steel beam span 24 feet for a pergola?
I wouldn't bother with an architect.

Hire a structural engineer if anyone.

You want a 24' long beam, probably two?  with just some minimal loading from open air beams that sit on the two 24'L beams?  A quick diagram would help.

24' is long but I've done wood PSLs 5.25"x11.75" that open air span more than that and support floor and roof loads.