It is a stylistic tradition which I notice in Renaissance literature in Italian and French. It was apparently considered more elegant, and more polite to offer certain verbal praise in the plural: grazie mille, merci mille fois, many thanks, salutations, greetings, saludos, congratulations, félicitations, my apologies, and so on. It's in all the Romance languages, and English, which is an adopted half-member of that family.