How to report about how many users have registered in a given timeframe?
Here are some modules to consider, for anybody who is not able or willing to write some PHP coding to answer such typical reporting question (or anybody who does not want to enable the PHP filter in a site):


The Forena module can be used for such types of reporting. It comes with all sorts of Sample reports and solutions, such as the Site administration reports, which contain data about the actual Drupal site. Here are some details about those reports: 


Active Users - A variation of D7's admin/people, including hyperlinks to the Logs for User report.
Active Users Email - A demo of the ability to send email to all active users of a drupal site.
Logs for User - Most recent watchdog log entries for a user (specified as a report parameter).
Role Details - A table displaying all users for a Drupal role (specified as a report parameter), and the Drupal permissions granted to them, with for each user hyperlinks to either view or edit the user's profile.


Cloning these sample reports into custom variations is straight forward. E.g. tune the active users report with some report parameters to only report about user registered this week.
For somebody looking for info related to visitors of a website (registered users is a subset of them, no?), the Visitors module might be a valid alternative to consider also (and/or addition to the above module). Obviously, to submit a registration request one has to "visit" an URL like user/register (or something similar).

The demo site contains some sample reports (charts) available to anonymous users also. This demo site also shows some additional blocks with reports also created by the Visitors module module. Refer to the Visitors community docu for some more details.

Extra reporting features can be enabled via these sub-modules:


The Visitors GeoIP module, to also report about countries and cities.
The Visitors Browscap module, providing statistics on visiting browsers.



Both modules also support charting of those data (a typical related requirement ...)

Disclosure: I'm a (co-)maintainer of Forena, and the maintainer of Visitors.