What is your estimate on the number of men Daven Lannister took with him to the Siege of Riverrun and what is your rationale for this figure? Additionally, how many men do you believe Volantis is currently sending to Meeren? Forgive me if you’ve answered these before.

According to Jaime IV of AFFC, the Freys at Riverrun have 2,000 men and they outnumber Daven’s contingent. So I would say anywhere between 500 and 1500? 

As for Volantis, they had 6,000 men just to guard against the Golden Company in Griff I, and they had 300-500 ships according to Victarion. so I would say anywhere between 10-30,000 infantry on those ships (30-100 per ship plus the normal crew).