Excel

I've got a list of project names running down one column, in another column I have numbers against each of these projects numbered from 1 to 87.
Some new rows have been added, in different places, so now I have numbers to put in against those new rows that have been inserted but. I don't want to repeat or use a number that already exists so how do I .. give those new rows a number without repeating one that already exists. I.e ? filtering showing all my numbered rows (in order) and then having, directly below it, the rows that have no numbers. Thus enabling me to continue the number ordering.

Thanks
Joan