Se OP desidera includere informazioni aggiuntive (aggregazioni aggiuntive, ecc...) utilizzando i dati di users
tabella:
SELECT `users`.`id`,
count(`receipts`.`uId`)
FROM `users`
INNER JOIN `receipts` ON `users`.`id` = `receipts`.`uId`
GROUP BY `users`.`id`
ORDER BY count(`receipts`.`uId`) DESC
In caso contrario, solo le receipts
la tabella è obbligatoria...
SELECT `users`.`id`,
count(`receipts`.`uId`)
FROM `receipts`
GROUP BY `receipts`.`uId`
ORDER BY count(`receipts`.`uId`) DESC