Don't let the discontinuation date lead you astray. Apple made them into early 1991 (I've seen several and own one myself to verify this).

I would say between the regular and FDHD/SuperDrive models there are a good deal of SEs out there. Remember there are many hidden in closets, attics, etc. (I know someone with one in his garage that hasn't been turned on in years).

The actual figure? I have no idea but the SE was in production from late February 1987 until January-February 1991. Maybe around 700K or so for the entire run?