Forum:Magical Dimension help

Hi, i wonder if you can activate "Magical Dimension" when you have 2 or more spellcasters on the field? My friend says you can only activate it when there's 1 spellcaster on the field becouse the card says "A Spellcaster" but if they would mean 1 spellcaster wouldnt the card say "One spellcaster" then? or am I wrong?

thnx! // Eric