Si OP souhaite inclure des informations supplémentaires (agrégations supplémentaires, etc.) en utilisant les données des users
tableau :
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
Sinon, seuls les receipts
le tableau est obligatoire...
SELECT `users`.`id`,
count(`receipts`.`uId`)
FROM `receipts`
GROUP BY `receipts`.`uId`
ORDER BY count(`receipts`.`uId`) DESC