How many Christian religions in the world?

There is only one Christian "religion." The different groups are called denominations. They are not other Christian "religions."

Christianity is the act of following Jesus the Christ, having accepted that He is God's son, come to earth to pay for sins (actions that we have done to separate ourselves from God) by dying for us even thought he was blameless, then rose from the dead.

There may be different denominations that interpret Scripture slightly different in the way that we are to best follow Jesus, but the core is the same. Therefore, they are not different religions.

Religions that are not considered Christian, but seem similar are Mormon and Jehovah's Witness.