iPad: Newly installed apps don’t show up on Home Screens
There's an upper cap on the maximum number of Home Screens that can be created on an iOS device. The count is a bit higher for iPad than for iPhone.

Once you hit the limit, any newly installed app doesn't show up on Home Screen, as no more screens can be created. You can still continue to install new apps as long as free storage space is available on your iPad.

You can search for an apps using Spotlight even if its icon is not visible. One way to view more apps is to organize apps within folders. A folder can house many apps (the limit varies between iPhone and iPad). Since a folder only takes up the visible are required for one app, you can see more apps when using folders than without one. (The maximum number of Home Screens or maximum numbers of apps in a single folder for a device are not officially documented and can be determined by experimenting.)

To create a folder, simply tap and hold on an app icon until all the icons start to wiggle. Now drag and drop an app icon onto another to house them into a folder. You can also assign a custom name to the folder to easily recognize it. However, you can not create a folder inside a folder.

Once you have organized apps into folders and are able to see the icons, you can drag to organize them on Home Screens as desired.

With iPad drag and drop support introduced with iOS 11, you can also quickly move multiple apps around.

Alternatively, if you have access to a Mac or a PC, you can also re-organize apps icons easily using iTunes.