The copies are created on the stack, so they’re not “cast.” Abilities that trigger when a player casts a spell (such as the Storm’s ability itself) won’t trigger.

Each Storm’s triggered ability counts only how many times you’ve cast your commander from the command zone before that triggered ability resolves. If you cast your commander again later, you won’t get another copy.

The amount of life to lose is determined only as Skull Storm resolves. For example, if an opponent controls no creatures and has 30 life when you cast Skull Storm and make two copies, the first to resolve causes that opponent to lose 15 life (so they have 15 life left), the second causes that opponent to lose 8 life (7 life left), and the original spell causes that player to lose 4 life, leaving the player at 3 life.

Each Storm’s triggered ability can copy the Storm spell even if that spell is countered before that ability resolves.

If your commander was countered, that time you cast it still counts.

If you have multiple commanders, you’ll copy this spell for each time you’ve cast either of them. For example, if you’ve cast one commander once and one twice, you’ll copy this spell three times.