Depending on what your water supply is near your safe, 1 or 2 heads might do it. A standard commercial 1/2" pendant sprinkler supplied with 7 P.S.I. from a minimum 1" line will put down .10 G.P.M. over an area 15'X15'. However most homes aren't piped with water lines that big.
You would need to determine your available piping size, pressure in the line & the area you would like to cover. Also look at what is in the area that might be considered an obstruction to the flow pattern developing. Once you have that basic information you can look at choosing a head and placement.
I would be happy to give you advice if you'd like to post pic of the area and dimensions of the safe you are considering.
If you haven't figured it out yet this is what I do for a living.