The arithmetic of transfer days, and why three islands in a week is the most common planning mistake.
Give every island three nights and you get two full days there. Give it two nights and you get one full day, because arrival and departure days are both partial. One night is a stopover, not a stay.
That means a seven-night trip supports two islands, ten nights supports three, and fourteen nights supports four comfortably or five if every hop is under an hour.
Naxos to Paros is 35 minutes. Gili Air to Lombok is 15. Where hops are that short you can drop to two nights on an island without losing a full day, which is how the 14-Day Greek Island Explorer fits five islands into two weeks.
Where hops are three to six hours — El Nido to Coron, Koh Lanta to Koh Lipe — never go below three nights.
Nights available, minus one buffer night per week, divided by three, rounded down. That is your island count. Anything above it should be justified by a hop under an hour.