There can be various reasons why you are missing some of your Keys.
It might be because you have pressed "Next" on the Wordy Weekend event.
That consumes keys to skip the character. That is why there are keys displayed under "Next".
Also, be aware that it requires Keys to use the ‘Save Me’ option and that the cost doubles every time you use it. For example, the first time you use the feature, it costs 1 Key, second time 2 Keys, third time 4 Keys, fourth time 8 Keys and so on.
It could also be that you have accidentally purchased something in the game requiring Keys.
If you didn’t purchase anything, someone else might have access to your device (or another device using the same Google Play Games, Game Center, or Facebook account) who might have spent Keys.
Unfortunately, purchases made with Keys can not be reversed. We recommend that you are cautious when you navigate in the game before you click on buttons with the ‘Key’ icon.