Configuration of a Query-Statement
I do not think that Max Results takes a local variable. However if you used the ~ notation (Tilde), I think ~GCVName~ would work, since it is replaced at driver start as a sort of search/replace in the text of the rules.
But also, I doubt it means what you think it means. Alas, it does not mean only return 250. Rather it means return query results in pages of 250. So if there are 1000 results, you will get 4 pages of 250, not the first 250 only.
(Max results PER PAGE would be a better name for the field than the current value, alas).