how to get an accurate number of frames in tcl script?

I have a trajectory contains 75 frames (*.dcd file). But when I try to calculate the number of frames by the tcl commands:

set id [molinfo top get id ]puts [molinfo $id get numframes]

I get much lower number, namely 3.

so how can I get the correct number of frames?

P.S.: I know that I can use VMD for this purpose, but the trajectory file is on a remote computer, and I need to run several calculation, like RMSD, that depends on the frames number through a code(without graphical interface).