I am certain you will find many people here who will say that for each gift sent a thank you card should be mailed.
My thoughts are to let her write out those all-encompassing thank you's!! What the heck! Time is precious. WHat is the point in buying/writing more cards, buying more stamps, etc. It's sad but these days I'm glad to get any thank you notes at all. It wouldn't bother me one bit if I got one that was for more than one gift to the same recipient.