1. The number of Christian denominations is stated at around 33000-39000. Well, it really depends on how you look at it. Presbyterian Church in Singapore and Presbyterian Church in Malaysia will be listed as two different denominations, so sometimes it is simply due to technical and geographical issues and not for more despicable and abhorred reasons such as power-seeking and wealth-gaining. So different people will look differently at this number. Personally I will classify them according to the twelve major branches: Catholics (Latin rite and Eastern rite), Eastern Orthodox, Oriental Orthodox, Assyrians, Lutherans, Anglicans, Calvinists (Presbyterian/Reformed), Anabaptists, Methodists, Baptists, Pentecostals, Charismatics. The number is coincidental and has nothing to do with the tribes of Israel or the disciples of Jesus. No need to waste your time to find the Judas Iscariot among the twelve of them.
2. An important litmus test for church unity is the sacraments. Do you acknowledge the baptism held in other churches? Do you require those who have been baptized in other churches to be re-baptized again when they want to be members of your church? Who can participate in the Eucharist? Do you allow those who are baptized in other churches to take the bread and the wine? For which churches would you answer Yes and for which churches would you answer No (by the way, the answers don’t need to be universal for all questions; some could be Yes and some could be No)? For example, personally, I have been barred once from receiving the Eucharist in the Catholic church and it does show basic disunity among us.