Sed runs as standard equipment on modern Unix Systems as well as has been ported over to dos. It is recommended that you use the GNU version if possible as it is the most easily accessible thanks to the open source movement.
Hmmm... difficult question to answer. Perhaps to get a real snapshot of the total usage statistically one would need to determine the number of people using Unix based systems excluding dos. Then one would need to add those programming in perl which would mean pretty safely millions anyhow...
This one is pretty difficult to answer fully as i tend to influenced by many forces on the Internet at the time of writing. From Larry Wall of Perl fame who continues to amaze me. As well as the authors and experts of sed everywhere!
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