Auto number items that are pasted into spreadsheet

Hello,

I need help please with auto numbering.
Perhaps there is a built in feature that does this.

I need a formula that can be pasted into a cell which looks to see which number precedes it in the same column.
There will be empty cells between these numbers.
It should add 1 to the previous number in the column to determine its own number.
Should there not be a preceding number it will call itself 1.

Thank you!
B

Assuming the formula will be pasted into A2 (data starting in row2), then copied down:

=IF(B2<>"", MAX(N(A1), MAX($A$1:$A1))+1, "")

This will only put numbers in column A if there is a value showing in Column B, and it will increment as it goes down.