While people believe that Alucard is immortal, he says that he isn't, and we are later shown that this is because the only reason he is back from the dead is because he uses the enormous supply of souls he has absorbed throughout his unlife. However, at the end, he may have gained the ability to revive freely thanks to Schrodinger's powers.
By saying that he is not immortal, he may have meant that a being that is immortal doesn't mean it would be capable of handling such a fate and would wish to be killed. This side is greatly shown in the series where even Alucard himself wishes to be killed though by a person he sees as worthy.
Based on the current information, the only time Alucard was said to be mortal was when he released every soul inside him as an army, which made it impossible for him to use them to revive himself. However, this happened before he gained Schrodinger's powers. So unless more information is shown, the best guess is that Alucard can revive without limit now (like how Schrodinger did when he was shot 2 times but didn't die and reappeared fully healed, only to die when he willingly sacrificed himself in an effort to kill Alucard).
To know more please check the wiki. I am assuming all of this based on the information given and my own ideas. However, I believe this is the best description of Alucard's immortality as of yet.