When Ron says '3 to go'
I read the books long ago (and have probably forgotten more than I remember) and have seen the movies multiple times but there's something I don't understand.
After they destroy the locket with the sword, Ron says something like 'Well, 3 to go' but how did he know that there were only 3 left?
At this point they had destroyed Tom Riddle's diary, Dumbledore destroyed the ring, and now the locket. They know there are 7 horcruxes - shouldn't Ron have said '4 to go'?
Any insight would be appreciated !