The full number of the elect church could have been from the nation of Israel. (Exod. 19:5, 6), but Rom. 11:7 tells us that the nation failed in their faithfulness. “What then? What Israel sought so earnestly it did not obtain, but the elect did. The others were hardened…”
The call to be in the Church was opened up to the Gentiles. Rom. 11:25-33 elaborates on this, “…I do not want you to be ignorant…my brothers (in Christ),…Israel has experienced a hardening in part until the full number of the Gentiles has come in…”
The book of Revelation is full of symbols, and this passage is no different. Here, the twelve tribes of Israel are symbolic of the complete Church class. You will find this to be consistent throughout the book of Revelation.