If the numeric sequence starts in row 1, you can put this formula in a column and copy down to get sequential numbers:
=ROW()
If the numeric sequence needs to start row5 but start count at 1, put this in row5 as the first formula, then copy down:
=ROW(A1)
You can use VBA to enter that formula. Let's say you have a random number of rows of data and you want to put numbers in column A, there is existing data in B:E. Look UP column B to spot the last row and store that number in a variable.
or:
Another way using OFFSET to answer your final question:
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