So are you seeing too much forms when you are using Facebook Lead Forms in your ads?
The number of Forms can add up as each time you want to correct a typo, you need to create a new one (I typically duplicate the form I need to correct).
You cannot really delete it. Though one can archive it.
Go to your Form Library by entering this url:
Replace <YOUR-PAGE> with your Facebook Page url.
So first go to your Facebook Page (where you have admin rights).
Next go to Publishing Tools.
Next click on Forms “Library” under “Lead Ads Forms”.
There you can first select the Forms you don’t want to see any longer. Next you click on actions and archive it: