Yes, gwendolynaoife, you can send in as many as you want at a time (so long as they are in multiples of 5). I've sent in 100 once before - sent them Priority Mail with a tracking number (and return receipt!). They don't speed up the process of crediting the miles based on how you send it (so there is no advantage to overnight mail) - but at least I felt comfortable that they had been received once I got the tracking confirmation.