The number of apps that you have installed on your iPad is shown on it in Settings > General > About > Applications
Or when connected to your computer's iTunes, select the iPad and hover your mouse pointer over the Apps section of the storage bar at the bottom of the screen, that should display the number of apps (which will hopefully be the same as the number shown in Settings above).
For the difference between the number of apps shown in the Apps part of your iTunes library and how many are in the directory in windows explorer, it's possible that for some of the apps that you have in iTunes you no longer have their .ipa files on your computer - if you no longer have their .ipa files then iTunes won't be able to install them if you select them for installing.