I normally build 3 each appliance for the 15 day goals and 6 of each for the 5 day goals. I always manage to finish the goals and am convinced that planning when to start recipes is the most important factor in these goals. Like you, my reasoning for building a set number of appliances is completely arbitrary and I honestly can't recall how or why I started doing them this way. All I know is that I've been building goal appliances in 3s since the goals started to occur with any regularity way back when.
That said, when an appliance requires so few parts (like these Folk Ovens) I will build 10 of them just for expediency's sake. Seeing as how I never need to finish these 5 Day Goals, my personal aim is to unlock the new recipes as quickly as possible. I really aim to have as many days of downtime as possible before the next goal starts.