You should be able to get a rough square footage estimate on the walls (1400 sq ft of floor space is not esp relevant) for each color you plan to use. The sales clerk should be able to use that to estimate the number of gallons you need for a first coat.
Second coats take a bit less paint than first coats, but not a lot less. Whether you need a second coat will depend on what color is underneath the color you paint. For example, an off white on an off white will usually cover well in one coat. It is hard to judge the coverage on a first coat until it is dry and you examine it in a good light.
I find it is cheaper to underestimate the paint a bit and allow for a second trip to buy whatever you need to finish up. Rollers are a lot cheaper than paint and unused paint is just a kind of toxic waste you need to figure out a good way to dispose of. I hope that helps.
― Aimless, Sunday, 22 December 2013 20:14 (ten years ago) link