Seeing as how everyone else has argued about Alucard's immortality, I may as well throw my hat in the ring. Here is what I think of Alucard's immortality. It's not the soul count theory, but another theory that makes more sense (to me anyways). Alucard, and all other true vampires, don't get their regenerative powers from the souls they've taken in, but the energy they acquire from drinking blood. Vampires lose the energy they get from blood by a) not feeding/starving, or b) having their energy used up through using abilities that demand alot of power, such as releasing souls, or regenerating from what would be fatal hits. The souls themselves are just another ability, but require alot of energy to be released, depending on how many souls are in a vampire, if there's just one, in Seras's case, then not much, but in Alucard's case, then alot. Other abilities tax their energy reserve as well, such as disengaging their body to turn into that shadow monster thing that Alucard turns into at times. Unleashing familiars, however, is the most taxing ability a vampire has, as most abilities, like superhuman strength, sight, basic regeneration from gunshot wounds or small cuts, etc won't tax a vampire at all, it's only the unleashing of souls, disengagement of your physical body, or regenerating from wounds deemed fatal to a vampire that will call upon using up that energy. When Alucard released all of his souls, he had a considerable amount of energy left, but he was nowhere near where he was before, and as his battles with Anderson and Walter progressed, his energy level was started to dwindle (probably a reason as to why he started to drink from all of London's dead, to restock). See what I mean? Make sure to tell me what you think.