This has been tested in both creative, singleplayer and on a mp server. When multiple anvils fall onto a non-solid block, such as a chest or torch, more often than not, only 1 anvil will be picked up.

It seems to be something to do with stack combining in-world, since you will get maybe 2 or 3 back if the dropped anvil(s) fall off the side of the hitbox of the non-solid block while the others are still falling. But if all of the dropped anvils fall onto the hitbox, then they all merge into a single dropped anvil. 

A similar issues seems to have been reported some time ago, but resolved here: https://bugs.mojang.com/browse/MC-109610 . So I am reporting it again, since it is occurring anew.