How many co-expressed genes would be expected in a tissue?
Check the beautiful publication of Daniel Ramsköld et al. 2009, which holds the numbers for generally anticipated co-expression.
The specific level of co-expression, which applies to your scenario, will depend upon your tissue, your thresholds, and your definition of co-expression.
It you look for a co-change of some genes across different specimen (rather than co-expression), the number of genes that should change will depend upon the underlying biology (and thus prevent a general answer without considering the specifics of your specimen).