99 Tahoe fluid chng?

Hello, I have a 99 Tahoe(2WD) and I want to drop my pan and chng my filter. About how much fluid does it take to replace what was drained, im thinking about 5 quarts? Also, if I maintain this procedure in maintaining my , about every two years, as opposed to getting a complete flush, will it hurt it? I use a bottle of that Lucas additive whenever i change my fluid, thnx...