They will still lay them along the top as best they can,
Or If the underside to your cover is accessible to them they will lay on it,
yes upside down,
A single clutch can have hundreds of eggs, but the chances of all surviving is highly unlikely, unless you over feed and have alot going to the bottom..,
Also some fish pick at them, some even eat them..
If you don't want them hatching in your tank,
you can gently remove the clutch, place it in a bowl with a lid, sit the clutch on a moist paper towel and another moist towel on top, close the lid and check it everyday to make sure its just moist and not soppin wet.
heres a picture,