Yes you can review as many different apps at one time as you like. 

Login to your App Review website and on the main dashboard page is the app review database. You can scroll through and choose an app that you would like to review and then download it to your device using the download button for the Apple App Store or Google Play Store. 

After you have tested that app you can log back in to your website to write the review for that app by clicking on the 'Write Your Review' button in the app database for that app, or you can select another app from the database to download and test.