A single icon can represent any number up to 9999 (see bottom row). It's not hard to decipher. Break the icon into four quadrants, then read the bottom left, then bottom right, then top left, then top right.
I have no further information on the system, or its name. The via at Reddit indicates that it was developed by Cistercian monks and was "used for years, divisions of texts, the numbering of notes and other lists, indexes and concordances, arguments in Easter tables, and even for musical notation."
This system is explained - somewhat slowly - in a Numberphile video (hat tip to reader Keith).
Related: Hieroglyphic numbers