With each App Engine account that Google validates, it gives you 10 applications that you can create under that account.
An example screenshot of the App Engine Admin console for my account is shown below:
There is no way you can get more than 10 applications under a single account. You might be forced to use some other email address / mobile phone number if you need more than that.
On many occasions, it is noticed that developers get excited and created application IDs that they may not need. If you end up with that situation and need more Applications, then the only recourse is to delete the existing Application ID. Remember that once the Application ID is deleted, there is no way to retrieve it back.
Hence use the following guidelines:
1) Think carefully about your Application ID. It should reflect what your application is doing.
2) Keep in mind that you have just 10 Application IDs, hence do not end up exhausting them and then end up deleting some of them to free up a slot or two. You might regret losing out on some unique names.
Check out all the App Engine Development Tips.