Green’s Dictionary of Slang

penny number n.

[the price of the magazines]

a sensational story, serialized in weekly penny magazines.

[UK]J. Greenwood Seven Curses of London 179: ‘It was them there penny numbers what I used to take in, sir,’ or words to that effect, and the little humbug is rewarded by a pat on the head.
[UK]‘Doss Chiderdoss’ ‘Penny Numbers’ Sporting Times 11 July 1/3: ‘Penny Numbers’ [title] (There is no diminution in the number of youthful culprits whose wrongdoings are attributed to the perusal of sensational literature.).

In phrases

by/in penny numbers (adv.)

in instalments.

[UK]Sporting Times 22 Mar. 1/4: The going down was all right. Went down on a dogcart, then on Kyber, then on Hugger Mugger, then on George, then on Magic, and then on Eyrie—I got back to town in penny numbers!