I would like to compare
activity or statistics for some sites let's say wordpress and drupal.
Is there any way how I can do that?
I was looking for some "number" on within the site itself....
If all you want to see is today's snapshot, just look at Stack Exchange's list of sites. For example:
If, however, you want something deeper, then you may want to check the Data Explorer. I wrote this query to dump the number of questions per day. I downloaded a CSV for both sites and then plotted their moving averages against each other:
There's a wealth of raw data available for usage statistics across the SE sites.
The Stack Exchange hub has a list of the SE sites, along with basic statistics for them all. Beyond that, moderators and SE staff have access to such information, but it is generally not disclosed to the general public without good cause.