That is never actually stated. Moiraine states that the thirteen weakest Aes Sedai could shield any male but there is a minimum strength threshold to be allowed to take the final test, so the weakest Aes Sedai are reasonably strong (e.g. Morgase so weak she could not touch the source consistently, and Sorilea stated to be too weak to be allowed to test). There is nothing special about there being thirteen other than that is the largest circle possible with all females (it would be stronger than 12 but not by as much as the 12 would be stronger then 11 - there are diminishing returns for unbalanced circles). Also all in character statements (even by non-black Ajah Aes Sedai) are subject to the unreliable narrator issue - an Aes Sedai can state as truth what she thinks is true even if she is wrong.