The number of the Pillars of Islam depends on which faction of Islam the Pillars relate to.
- The Four Pillars of Islam (Abul Hasan Ali Nadwi)
- What Are the Five Pillars of Islam?
- Six Pillars of Islam
- The Seven Pillars of Islam: The Esoterics of Walayah
- Ten Pillars of Islam
Those are very real differences in the number of Pillars of Islam. The differences cause the ongoing tensions and strife between various factions of Islam. The most important difference is perhaps whether a given faction of Islam considers the obligation to include or exclude the Pillar of Islam that pertains to the importance of the Jihad. Here is a Wikipedia article that attempts to explain those differences: https://en.wikipedia.org/wiki/Five_Pillars_of_Islam
It is very important to consider the gravity of including or excluding the Islamic Jihad in the numbers of Pillars of Islam: https://en.wikipedia.org/wiki/Jihad
Consider, if a faction of Islam subscribes to the Islamic Jihad and kills other Muslims because they ostensibly need to be killed according to the premises of the Islamic Jihad (some do that, obviously, just look at Muslims killing Muslims in the Middle East), then there is nothing for the members of a faction of Islam that subscribe to killing other Muslims (there is more than one faction that does) that will prevent them from wanting to kill anyone who is a Christian, a Jew, or who belongs to any other religion or holds no religion at all.