This would only be a mild, temporary barrier to cracking. The only way such a hash would even be useful would be if the application checking the hashes had a hard-coded or algorithmic way to determine the number of rounds. If so, that software could be inspected and/or reverse-engineered and the number of rounds could be extracted and applied - so leaving off the number of rounds wouldn't provide much protection.

Most hash formats that use rounds include the number of rounds as part of the hash itself (the "1000" in this example, taken from [the list of hashcat example hashes][1]):

    sha256:1000:MTc3MTA0MTQwMjQxNzY=:PYjCU215Mi57AYPKva9j7mvF4Rc5bCnt

The easiest workaround is to simply generate each hash:

    sha256:1:MTc3MTA0MTQwMjQxNzY=:PYjCU215Mi57AYPKva9j7mvF4Rc5bCnt
    sha256:2:MTc3MTA0MTQwMjQxNzY=:PYjCU215Mi57AYPKva9j7mvF4Rc5bCnt
    sha256:3:MTc3MTA0MTQwMjQxNzY=:PYjCU215Mi57AYPKva9j7mvF4Rc5bCnt
    sha256:4:MTc3MTA0MTQwMjQxNzY=:PYjCU215Mi57AYPKva9j7mvF4Rc5bCnt
    sha256:5:MTc3MTA0MTQwMjQxNzY=:PYjCU215Mi57AYPKva9j7mvF4Rc5bCnt

... and then feed *all* of them to your cracking software.

How confident are you that that is the hash type? Did the number of rounds accidentally get truncated during collection of the hash? Otherwise, it would be pretty unusual for a developer do this. It would have to be a local/custom application - and a very naive one at that. 

No well-known application omits rounds in this way (that I'm aware of). 

  [1]: https://hashcat.net/wiki/doku.php?id=example_hashes