JWs believe that only a literal 144,000 "true" Christians will go to heaven. They refer to these folks as "anointed". They believe that the Eucharist was established as a covenant to rule as kings and priests with Jesus. Therefore, only the individuals that are anointed are allowed to take the bread and wine. That number is supposed to include all of the anointed that have ever lived, back to the apostles.
As you can see, the math likely doesn't add up and obviously the scriptures having to do with that number are symbolic (if you believe in the Bible which is appears you do).
Most elders are not anointed and there are only about 18,000 or so that partake in the emblems each year. I'll let others go into more detail.