How space scientist keeps the track of the stars? In billions of stars, how space scientist keeps the track of the stars ? 

There is a star called KIC 8462852, I saw that picture, star is so small from the earth. It has higher chance to miss that star in the universe.
Scientists make lists of stars in a catalogue. In the past this would be in a book nowadays it is more likely to be a computer database.

The stars don't move (much) so it is possible to make a list of them by position Modern catalogues are online (see SINBAD older ones were on paper (see the 1918 Henry Draper catalogue)

The Henry Draper catalogue is typical and easier to follow. It is just a table, the first column is the catalogue number (used instead of a "name")  the second column is a cross reference to another database. Then there is information on the location of the star in the sky using the RA and Dec system. Next comes information on the brightness of the star (estimated in two ways). Some letters come next to describe the spectrum (or "colour") of the star and finally some short "remarks", and a "plate number (the number of the photograph that contains an image of the star"

Scientists need to be systematic and careful to avoid missing anything.