The best way to get an accurate count of new users is by creating a chart and adding it to your dashboard.
First, click on charts in the admin navigation and then add a chart with the following fields selected.
Title: New Users
Data: Users
Options: New Users
Counting Options: Running Total
Show on Dashboard: Enabled
Time scale: Days or Weeks (depending on your situation)
Play around with the chart feature to see what kind of charts you can make. You may hit the preview chart to see what it looks like before you hit save and have it appear on your dashboard.