With this in Numbers:

Untitled.numbers.zip (55.5 KB)
…and assuming the sheet is open in Numbers, this works for me:

[test] Search for Number and Return Adjacent Cell in Numbers.app.kmmacros (3.4 KB)
Notes:
- In the sheet, the cell with the “unique numerical number” has to be formatted as number. If it is formatted as text, remove the
as numberin line 3 of the script. - Not tested with a large dataset; maybe it is too slow.
- If the unique number is always in the same column, you could limit the search to that column.
The contained script:
tell application id "com.stairways.keyboardmaestro.engine"
set inst to system attribute "KMINSTANCE"
set targetNumber to (getvariable "Local__Search for Number" instance inst) as number
end tell
tell application id "com.apple.iWork.Numbers" -- Numbers
tell document 1
tell active sheet
tell table 1
set theMatch to (first cell whose value is targetNumber)
set {r, c} to {row's address, column's address} of theMatch
tell row r to set adjacentCell to cell (c - 1)
value of adjacentCell
end tell
end tell
end tell
end tell