I was unable to find a exact number of devotees to Shakism also but there are many. Here is what I found below, I stared your question so that perhaps one of my Hindu contacts can give you a percentage. India is considered to be the land of the Goddess in these modern times.

I also consider it a great misfortune for many human beings that the Goddess is rejected in many religions today.
Shaktism is practiced throughout the Indian subcontinent and beyond, in countless forms, both Tantric and non-Tantric; however, its two largest and most visible schools are the Srikula (lit., family of Sri), strongest in South India, and the Kalikula (family of Kali), which prevails in northern and eastern India