There weren't even 12 tribes then. Judah was still distinct (Judah had to be distinct in order for Christ to be of the tribe of Judah), but like nine of them we gone (they all dissolved into each other and became the Samaritans) by then. The closest thing to a church of the 12 tribes were the 12 Apostles themselves, as representatives of the 12 tribes.
Just because the kingdom was split does NOT mean that the number of tribes was not still 12.
The divided kingdom will be rejoined in the future, once God removes the body of Christ from earth and begins dealing with Israel again.
Ezek 37:15-20 (AKJV/PCE)
(37:15) ¶ The word of the LORD came again unto me, saying, (37:16) Moreover, thou son of man, take thee one stick, and write upon it, For Judah, and for the children of Israel his companions: then take another stick, and write upon it, For Joseph, the stick of Ephraim, and [for] all the house of Israel his companions: (37:17) And join them one to another into one stick; and they shall become one in thine hand. (37:18) ¶ And when the children of thy people shall speak unto thee, saying, Wilt thou not shew us what thou [meanest] by these? (37:19) Say unto them, Thus saith the Lord GOD; Behold, I will take the stick of Joseph, which [is] in the hand of Ephraim, and the tribes of Israel his fellows, and will put them with him, [even] with the stick of Judah, and make them one stick, and they shall be one in mine hand. (37:20) ¶ And the sticks whereon thou writest shall be in thine hand before their eyes.