Hi Ista,
I think you looking for: n.rgroup? See help(latex).
Also, try posting your example in a copy and past form, like:
descriptives<-data.frame(n=..., mean=...) etc.
The easier it is for others to reproduce your initial results the
quicker the problem will be solved.
HTH,
Jon Loehrke
Graduate Research Assistant
Department of Fisheries Oceanography
School for Marine Science and Technology
University of Massachusetts
200 Mill Road, Suite 325
Fairhaven, MA 02719