[R] How to get the n (number of observations) per conditional group

[Dax]

Hello all,

I have a huge data set that I'm cleaning up a bit. I am extracted the
means per condition, but also need to get the n. Strangely enough I am
unable to find a function that could actually pull this off. I am
unable to use replications or count the rows using nrow.

What I am trying to obtain is the number of observations fitting
something along the lines of "data$total[data$spp==1&data$gyro==1&data
$salinity==0&data$day==i]"

Can anyone help me?