There's a good discussion about this
here
Quick Summary:
Lots of people like to run 7-front blocks to maximize number of attacks against most enemy units, since
ASF encourages a High Elf player to get in as many attacks in before the enemy gets to hit you back.
A 7x3 formation for Spearelves is a popular number as it utilizes the spears' extra rank fighting to full effect without the extra cost of unused combatants (really, any more than 20/21 elves just eats too many points in a single unit). The added rank bonus for a 4th row generally won't justify the cost,
IMO.
I believe the 7x3 is also pretty popular for Phoenix Guard for similar reasons as the Spears, though I personally like to run them at 5x3 or 6x3 or 5x4, just 'cause they tend to live through a lot anyways with their ward saves and benefit more from having SCR than attacks (purely my opinion here).
Swordmasters tend to be run in smaller units (7x1 or 7x2 or 6x2), namely because it makes the unit more efficient for its points (minimize the lack of no attacks from the back rows and the relative frailty of the unit - basically, you don't want them NOT breaking the enemy after the first round of combat).