Answer:
Quite simply really, Jehovah's Witnesses are taught to view the 144,000 and the great crowd mentioned in Revelation 7:4,9 as two different groups. However, this is not the case, first John is hearing the number (Rev 7:4) and then he looks and sees for himself (Rev 7:9). The number 144,000 is a symbolic number in the Jewish/Israeli culture, the number 12 is seen as a complete number, and the number 1000 is seen as a great number, much of the time to many to number. The reason why the number is 144,000 as opposed to 12,000 is because the context (of Rev 7:4) then goes on to speak symbolically of the 12 tribes of Israel, each having 12,000 each. Of course 12 x 12,000 is 144,000. It is quite obvious that the 144,000 and the great crowd mentioned in Rev 7 are in fact one in the same group located in Heaven.