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.
Thank you for your report!
We're actually already tracking this issue in MC-572, so I resolved and linked this ticket as a duplicate.
However, that ticket has already been resolved. That means that this is not considered a bug and won't be fixed. Please do not leave a comment on the linked ticket.
If you haven't already, you might like to make use of the search feature in the future to see if the issue has already been reported.
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