Hard to explain in short form, but basically this is how things work in a order book. When you go to buy you list a bid price and # to buy. Whenever a sellers ask price matches your bid the transaction will take place, but only for the # they listed to sell.
Hard to explain and I hope I am not just explaining something you already know. If you want to buy a set number, you can just click the red market buttin over the buy section when entering your order. You will not get the best price when doing this however.
The best thing to do is google how to trade an order book or something like that and bone up on it. 😜
Or...you can go to beeswap or tribaldex and trade SWAP.HIVE/LEO on the liquidity pools there.