I needed 10 watches but I realised that I had only 9. So what should be used:
1.) I am out one watch.
2.) I am one watch out.
3.) I am one watch short.
4.) I am short on one watch.
And if we're talking about a person:
1.) We're one player short.
2.) We're short on one player.
3.) We're out one player.
4.) We're one player out.
So what sounds natural #1,#2,#3 or #4 in both the contexts?
I am out one watch.
This doesn't fit your situation; I might use this if someone got into my gym locker and stole my watch.
I am one watch short.
I am short one watch.
Both of these work for the situation you describe (notice that I removed the "on" from #4).
Your #2 sounds more awkward than idiomatic.
Likewise, if we needed nine players for a baseball team, and we only had eight, we could say either one of these:
We're one player short.
We're short one player.
However, the phrase:
We're out one player
would make more sense if a player was on the team but then couldn't play for some reason (perhaps due to injury, or a red card).
