Et lille spejl means both a little mirror and one little mirror and it is true that using ét lille spejl would be stressing that it is only one mirror. You can never say "en lille spejl" though.
When you are counting, one can be both en and et (Both "en, to tre, ..." and "et, to, tre, ..." are correct), and what you should use, depends on what you are counting, e.g. "en kat, to katte etc. " and "et hus, to huse etc."
When you are simply counting, but not counting anything in particular, I'd say its more common to just use "en" as one, but I might be wrong on that.