"Israel" is the "spiritual" Israel of St Paul, Eddie, when he was turning a local Jewish sect into a global brand. The "Chosen People" (the Elect of the AV) are no longer defined by ancestry (and its physical symbol - circumcision), but by faith in the redemptive sacrifice of the resurrected Christ. So Paul was saying to non-Jews "You're in, too, guys, and on equal terms". This he referred to as "the circumcision of the spirit". Spiritual Israel includes all the original believers, both Jew and Gentile,
Now, having interpreted the Israel mentioned in the Revelation metaphorically the original inventor of what became The Watchtower Society, namely Charles Taze Russell, proceeded to take the number 144,000 literally. (Yes, don't ask, but read my forthcoming self-help monograph "The Ten Steps Needed to Create a Cult").
It was a limited offer, first come, first served . All the early Christians would be born-again members of the Elect, destined for Heaven". Over time the number grew, a few Arians here, a few other "heretics" there, a Servetus or two, and of course CTR and his early followers. I think the oligarchs who run the business and collect the profits are all supposed to be members of the group (referred to bin the literature as "the Remnant".
So the number 144,000 was reached probably early last century and the offer closed. It just leaves the occasional sale and return caused by a formerly chosen one apostasising. All the proselytising of the JWs since the time of its second CEO "Judge" Rutherford will be offering new converts the prospect of eternal life on earth, not in Heaven.