In Japanese you don’t need to change things to the plural form as often as in English.
If 複数 is multiple, and 形 is shape, then when something is in a "multiple shape," it has more than one version of itself. It is plural.
This is a jukugo word that uses the on'yomi readings of the kanji. You should be able to read this on your own. Will help to know the word 複数 as well.