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The "number of entries" allowed by a Schengen visa concerns entries into the Schengen area as a whole. Traveling directly from one Schengen country to another does not count as an entry into the Schengen area.
From Annex VII of the Schengen Visa Code:
- βNUMBER OF ENTRIESβ heading: This heading shows the number of times the visa holder may enter the territory for which the visa is valid, i.e. it refers to the number of periods of stay which may be spread over the entire period of validity, see 4.
When you travel directly from one Schengen country to another, you do not get an exit stamp or an entry stamp. You are not leaving or reentering "the territory for which the visa is valid."
Since Greece is not contiguous with the rest of the Schengen area, however, you must be certain that you travel directly to another Schengen country by air or by a ferry route that is not subject to passport control. If you have to pass through a passport checkpoint to leave Greece, you'll have to pass through another one to reenter the next Schengen country, and that will count as another entry.