Sequential numbering for pseudo-"Rows" tl;dr: Formula to number rows that doesn't break when some "rows" are made of multiple, merged spreadsheet rows.

Scenario: I am making a check-list (of things to look at when apartment-hunting) in Numbers 3 that involves some "nesting" by merging cells. It's more efficient to show than explain, so here's a little dummy:

  ||  A  |   B    |    C   |    D   |   E
-----------------------------------------------
-----------------------------------------------
1 ||  #  |      Item       |  Apt1  |  Apt2
-----------------------------------------------
2 ||  1  |   Sink works    |  [√]   |  [√] 
-----------------------------------------------
3 ||     |        |  Heat  |  [√]   |  [√] 
----     -        -----------------------------
4 ||  2  |  Has   |  A/C   |  [√]   |  [√] 
----     -        -----------------------------
5 ||     |        |  Gas   |  [√]   |  [√] 
-----------------------------------------------
6 ||  3  |   Door locks    |  [√]   |  [√] 


Problem: Normal ways of putting in a row number break because of the way Numbers handles merged cells. For example, A6=SUM(A5,1) would return 1, because Numbers thinks A5 and A4 are empty (whereas A3 contains 2).
Using way more math than should be needed, there is a formula that works. In the dummy spreadsheet above, you would put this in A3 and then copy it into all subsequent rows in bulk, just as you'd expect:

1 + ((    SQRT(8*SUM(  OFFSET($A$2,0,0,SUM(ROW(),-2),1)  )+1)    - 1) ÷ 2)