During mergers, you don't get to / don't have to buy any stations at all.
During conversion you have to buy exactly 1 station (when going from 2- to 5-share) or exactly 2 stations (when going from 5- to 10-). There's no way to buy more than that. The only way to buy less is if you run into the 8 token limit.
But the only way to get 2-share company into 5-share company is to merge it with another 2-sharer... Right?