None of them. I guess is the number of CGH probes from which the segmented window is composed. Remember that ABSOLUTE is for CGH and not for sequencing (although it can easily be used to analyse DOC signature in sequenging experiments).
My guess is:
"Chromosome": Chromosome number
"Start": Start position of the segmented window
"End": End position of the segmented window
"Num_Probes": Number of probes composing the segmented window.
If you use DNAseq data you should design windows of fixed size (or, better, windows of fixed mappability) and each of them is considered asd a probe.
Segment_Mean": Average log2ratio of signal intensity across the segmented window.
I suggest you use the HAPSEG package they suggest to perform segmentation. Otherwise I suggest the widely used DNAcopy, another R-package.