How many btu needed for 450 sq ft?

Check with the sales people in the store for your area and climate. I would say a 10 or 12,000 btu would do it, if you are only going to buy 1 unit.

In a typical residential application, your space is probably divided into different rooms. In that case, it is difficult to get cool air from one room to another. In that situation you would want multiple air conditioners. They usually do not come in a size less than 5,000 btu's per hour. So, to achieve enough total cooling, you will probably want two or more small air conditioners, one in the living room and one in each bedroom.