The correct is “πολλή ζάχαρη”.
“πολλή” is a female adjective meaning “a lot of, much”
“πολύ” is a proverb of quantity and it means “a lot”.
“Ευχαριστώ πολύ” = Thanks a lot.
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Vasiliki Baskou, Instructor/Director, https://learn-greek-online.com.