According to Official Guinness Records,
The most diamonds set on a mobile (cell) phone case is 6,576 diamonds and was achieved by Aaron Shum Jewelry Ltd (Hong Kong), in Hong Kong, China, as verified on 22 September 2018.The company was also awarded the record for the most diamonds set on a lipstick case on the same day.
For a complete list of 2018 records, please visit 2018 Guinness Records in China.