Alright alright here we go
So I’m wondering about Nightveil Specter ability if I do damage to an opponent and the exiled card is Doom Blade how many times can I cast the Doom Blade if I’m correct it’s as many times as I want as long as I have Nightveil Specter
Only once. If Doom Blade is a "car[d] exiled with Nightveil Specter" and you then cast Doom Blade from exile, it will no longer count as a "car[d] exiled with Nightveil Specter" since it's no longer in exile (C.R. 607.2a). (Note that in general, a spell goes to its owner's graveyard when it resolves, even if it was cast from exile [C.R. 608.2k].) In general, you then won't be able to cast Doom Blade again with Nightveil Specter even if Doom Blade returns to exile (unless it returns to exile again due to Nightveil Specter's first ability). See also this thread.
Also be aware that you may play the exiled card(s) for only as long as Specter remains alive. (rule 113.3d)
If Specter leaves the battlefield, the exiled cards are lost forever, even if Nightveil Specter later returns to play. (rule 400.7)
Furthermore, any card you play this way must be fully paid for, and its sorcery/instant timing must be obeyed. (as opposed to many similar cards, Specter does NOT pay for the cards you play this way, nor does it grant you the right to play a sorcery-timing card under instant-timing conditions)
So I’m wondering about Nightveil Specter ability if I do damage to an opponent and the exiled card is Doom Blade how many times can I cast the Doom Blade if I’m correct it’s as many times as I want as long as I have Nightveil Specter
So when you cast the exiled Doom Blade, you move it from exile to the stack. Eventually it resolves (or gets countered), and goes to the graveyard.
It is not in exile anymore, so Nightveil Specter doesn't let you cast it again.
If Specter leaves the battlefield, the exiled cards are lost forever, even if Nightveil Specter later returns to play. (rule 400.7)
Furthermore, any card you play this way must be fully paid for, and its sorcery/instant timing must be obeyed. (as opposed to many similar cards, Specter does NOT pay for the cards you play this way, nor does it grant you the right to play a sorcery-timing card under instant-timing conditions)
RULES OF MAGIC :
http://magic.wizards.com/en/game-info/gameplay/rules-and-formats/rules