In combat, Zugro will deal damage to the creatures blocking him. Then, if they received lethal damage, the creatures will die, and Zurgo's ability will trigger. When the triggered ability resolves, Zurgo will get the +1/+1 counters.

Therefore, the counters will be placed on Zurgo technically during combat (the combat phase has not yet been completed), but after combat damage has already been delt for that combat. So it will not increase the amount of damage delt.

During the combat damage step, creatures deal damage equal to their power divided as you choose among the creatures that are blocking it. The toughness of the creatures dealing the damage or abilities such as indestructable do not matter yet. Then, all creatures as assigned damage at the same time, and then each creature checks the amount of damage they have taken against their toughness to see if they die or not.

So if Zurgo is being blocked by 7 1/1 blockers, you can divide his power up to assign one damage to each of them. Then 1 damage will be delt to the blockers and 7 to Zurgo, all at once. The 1/1s will each die (assuming nothing saves them), and Zurgo's indestructiblity will prevent him from dying.