What do you consider "many"? Couple dozen is "many" for a small nation like Somalia that is pretty much a powder keg. The footsoldiers are many...war-afflicted uneducated young men who who want to belong to something. You need one true Al Qaeda trained leader for every couple hundred foot soldiers and you have an army.
Well, taking into account that a few men trained in Afghanistan managed to change the entire landscape of Somalia, 'a few' depending on whether you support Shabab could be thought of as 'too many', but it's asinine to believe that thousands of Somalis are in Afghanistan.
Is 700 Somalis in Lebanon fighting on behalf of
Hezbollah plausible?
It may have been a necessity for Godane, etc. to seek training elsewhere back when there weren't an abundance of training facilities in Somalia, but now? There's no reason for Somalis to be traveling abroad. You can completely rule out the thought of Al-Shabab sending foot soldiers to aid the Taliban. Even the most liberal of estimates' concerning Shabab's numerical strength wouldn't put them in a position to be throwing men at a quagmire a continent away.