When do you count your opponents cards to add the blast damage that Dash gets when attacking? Is it the number of cards when you declare attackers, when you focus to strike.... or some other time?

IMO... It would be a check at time of attack. As the card states..." While this unit is attacking alone..." emphasis mine. If not then, then your opponent could always empty their hand during edge battle (which isn't necessarily a terrible thing to force them to make a decision of giving him unit damage or tossing cards in edge that they may not wish to) to keep him from gaining any icons.\

Edited to say...
I stand corrected see below... should have stayed with my gut instinct and not over thought the situation!

The ability would resolve when Dash Rendar is focused to strike. It is best to use Dash Rendar to encourage an opponent to empty his or her hand, making it easier for you to win the next edge battle.

...if you look at the whole objective set, this makes more sense. You and your opponent use a whole bunch of cards to hopefully win the edge battle. Then, right before Dash Rendar strikes, you play one or more copies of Against All Odds, increasing the number of cards in your opponent's hand, thereby increasing your blast damage.

Blammo! Dash takes out an objective by himself.

Or you bluff your opponent with Lando and one un-tapped resource.

The ability would resolve when Dash Rendar is focused to strike. It is best to use Dash Rendar to encourage an opponent to empty his or her hand, making it easier for you to win the next edge battle.

Most of the time this is a correct answer, but misses some of the subtleties. The ability is actually always on and doesn't ever initiate or resolve. Dash has the blast icons from the moment he is declared as an attacker until the reward unopposed step (though the number of blast icons he has may change during that time). Most of the time, it will only matter at the moment he strikes, but it could also matter at other times, for instance if Echo Caverns is involved.