There were 8 sets of 6. ...48 postcards. However, there were a number of different printings, with many printing variations. Later series especially were re-printed many times.

THE 8 SETS ARE LISTED IN 8 SEPERATE LISTINGS ON THIS WEBSITE.
Series 71, 71a, 72, 73, 74, 75, 76, 79

Regarding the postcards publication year. The cards include illustrations from 'Bunny & Brownie', which was published in 1930, but also 'Fairyland'...and the A&C Black edition of 'Fairyland' came out in 1931. ...so I'd bet they were released around 1931, possibly to coincide with Fairyland's release in the UK ? Probably as a promotion, since they mention the Ida source-books on the reverse.

ELVES AND FAIRIES’ series of postcards. C. 1930’s. Published by A. & C. Black of London.

 

There are 48 full colour postcards. Eight sets of six cards. Each set of six originally sold together in an envelope.

These 48 postcards are very popular and are reasonably valuable, although there always seems to be a lot of them around.

The series is widely known about and has been well-documented in the past in various publications etc.

 

Regarding rarity, some of the eight series are much harder to find than others. Rarity seems to follow roughly in publication order. Series 71 cards are the hardest to find, closely followed by series 71a, then probably twice as common are series 72 & 73, then a fraction more common again is series 74. The most common series is series 75, second most common is 76 and 79 is about as common as 74. The variant series 79 cards with no white border are harder to find than any of the regular series.

Also, there is one card, one printing, that stands out as being exceptionally hard to find. That card is ‘Fairies were Dancing In and Out’, with a plain white border on each side. That must be the key card for the whole set.

The six cards in each series came in a printed envelope.

There are a handful of checklist cards with Ida Outhwaite pictures from the 48 cards...but with a checklist of several A&C Black card sets on the reverse, not for posting...
They are VERY rare.
...ANYONE WITH THESE CARDS, PLEASE CONTACT ME WITH INFORMATION...OR TO SELL THEM TO ME.
The 5 known checklist cards are photographed on this listing.