Even if 5 women created the shield, if you reduce it to just 1 woman to hold it, then the strength of the shield is that of the 1 woman holding it.

We see examples of this. Even though 13 women created Rand's shield, 6 were enough to hold it, but 3 were not.

So although you can reduce the number of women holding the shield sligthly, due to the difficulty in breaking through an existing shield, you cannot reduce it dramatically, as in to just 1 or 2 people, if the captive is much stronger than the shielders.

An average strength Aes Sedai simply cannot maintain a shield on Semirhage, even if it was created by 10 women. The number that created it makes no difference. It's current strength is dependent on the number of women currenlty holding it in place.