1. An image forming system comprising: an image forming apparatus, and an information processing apparatus which is configured to communicate with the image forming apparatus, the image forming system comprising: a processor; a forming unit that forms an image on a target in accordance with image data; memory storing computer-readable instructions that, when executed by the processor, cause the processor to provide: an acquisition unit that acquires factor information corresponding to a factor which is capable of causing variations in density of the image formed by the forming unit; a first determination unit that determines a number of marks in accordance with the factor information acquired by the acquisition unit; and a control unit that provides the forming unit with, as the image data, data pertaining to a pattern comprising a plurality of density marks which are different from each other in density and which are equal in number to the number of marks determined by the first determination unit; and a detection unit that detects a density of the image formed on the target by the forming unit in relation to the pattern, wherein the memory stores additional computer-readable instructions that, when executed by the processor, further cause the processor to provide a correction unit that corrects the density of the image in accordance with a result of detection performed by the detection unit, wherein the factor information comprises an index corresponding to a change in the factor; and wherein the first determination unit increases the number of marks as an amount of change in index increases.