Why only seven Immortals?

When Kemal tells the story about the origins of the Protector, he implies that there have been seven immortals since the time that the Protector was made, hundreds of years previously. One of the other Immortals even says that "an ordinary mortal" has never joined them before. Not even Faysal did this for all the years that he was the only one and at a disadvantage. But why not?

We certainly know that they are capable of creating at least one additional Immortal, or with the strategem that they employed near the end of Season 3, multiple Immortals. It also seems that having more unkillable soldiers for their army would certainly increase their chances of success against the Protector. And, up to a point, as we’ve seen, they would be able to control the new Immortals, so they wouldn’t need to fear a rebellion.

As such, why didn’t the Immortals add to their numbers in the preceding centuries?