When I use gnuplot with multiplot, I want to number these plots, but I do not know the way.
I can set the xlabel ylabel title for every plot, but
what is the way to number them as "1) 2) 3)" or "a) b) c)"
It sounds like you want to put a label on each plot; the command for that is
set label "1)" at graph 0.1,0.9
gnuplot does not do this automatically. The coordinates I gave as an example would put the label at the top left. For more info try help set label and help coord at the gnuplot command prompt.
EDIT: between plots you will probably want to unset label so that you do not get multiple labels in the same spot.