Loial states quite clearly that 6 Aes Sedai were unable to take Raolain Darksbane or Yurian Stonebow. And that was more than 1000 years ago. We know Aes Sedai have been growing progressively weaker as time went by, so those Aes Sedai would have been stronger than Aes Sedai of today.

From what we know so far it seems likely that Semirhage is approximately the equivalent of 3 average modern Aes Sedai.

Which would mean that 2 Semirhage's would equal approximately 6 Aes Sedai in strength. MODERN AES SEDAI, mind you, not the equivalent of the Aes Sedai in Darksbane or Stonebow's times.

So if 6 ancient Aes Sedai were not strong enough to take those two false Dragons, then 2 Semirhage's won't be strong enough to take Rand either. Not when he is at his full potential.

So you would in fact need 3 Semirhage's do the job.

In fact, Rand might well be stronger than Darksbane and Stonebow, meaning that 7 Aes Sedai would not be able to take him either. In fact, you may need 8 or 9 Aes Sedai to take him.

Meaning that Lews Therin was perfectly correct when he said it would take 3 Semirhage's and Mesaana's to shield him from the Source if he was already holding the Power.