Yeah, putting a real number on a dream to make it a little more down to earth is not arguing. Saving horses from slaughter is not all rainbows and butterflies. And he is right. You will need to have a very large bank account.
There are people who will ONLY adopt dogs from a Humane Society or Shelter, knowing full well that they can only take in one of the MANY dogs there. Some people do the same with horses. Nothing wrong with that.
If you are wanting to hopefully start a type of rescue organization, start talking with people who already have such organizations running. Maybe go work with one. They will be able to give you the best advice for starting.
If you are just wanting your personal horses to be slaughter rescues, then the number you will be able take in will be limited.