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Orcs have the advantage in attack and speed (the direct opposite of Necromancers), so you need to finish off the Necromancer before he captures his second town (the chance of getting units under his control is lower). I think that taking "Darkness Dampening" is a good idea (it won't help much against Blindness, though). In general, you should destroy half (or as many as possible) of the enemy troops before the Necromancer hero can cast spells (and the Boots of Speed tactic will help you with this). I usually play as Hagge, dividing the Centaurs into two groups (so that they won't all be controlled or blinded, and so on – there may be opponents who use similar tactics).
I can't say with absolute certainty about the number of summons, but they are random (and possibly have different probabilities of appearing – that's true). I can also suggest Maxim Sorokin's tactic (which is focused on speed)
http://heroes.ag.ru/heroes5/advices/12/ - here's the link.
The build depends on the map and the Necromancer's build. The most important skills are Attack, Leadership, Darkness Dampening (it's developed closer to the endgame, taken as soon as it's available), possibly Defense, Luck, and Logistics