Lookup (or IF) the next unique number based on a value of Y or N in another cell

i dont see a workbook from you to relate to, so assume your numbering is in column A, you would enter the 1st "seed" in A2 (assuming A1 has a heading), then copy that formula down. to make sure it only appears when something is entered in that row, you can tie it to column B (or any column that will have info in it), by wrapping it in an if statement...

=if(B2="","","="this is row "&MID(A1,13,2)*1+1) obviously you will need to change the "this is row " part, and also the ,13, i will explain that below

what ="this is row "&MID(A1,13,2)*1+1 does is as follows...
1st it strips out the text by getting the MIDdle string from A1 (the cell above), starting at character 13 (this is row =12 characters, +1 for the space), and returning the next 2 characters (change that to 3 if you will have more than 99 numbers)
then it takes the above - which is text, although it looks like a number - and multiplies it by 1 to convert the text number to a real number, and adds 1 to it
finally it concatenates (joins) the text with the increased number

hope that makes sense, if not shout