Why can we not say "un mil" for "a thousand"?
I googled "un mil" and several sources said "Just don't - it's wrong"
I think that is just what we do in English. We only use "a thousand" and "a million" and "a hundred" ... but we never say "a one, a two, a twenty" ... so I think in Spanish, none of their numbers are preceded by "a" , at least not a thousand.
Hay un mil estudiantes en total. I dont see why this wont work either.
Language is defined by the common practice of users and not its general rules. Pehaps numbers are an exception to the rule.