i am cosidering building a wall
7feet(w)x20 feet(h) on an exterior wall.
i am a beginner ,how or where might i find a diagram to set up
climbing paths?

Honestly i think that is enough holds. It all depends on the problems you are making and the feature of the wall. Is this vertical 20 feet? because a typical indoor route going that high has maybe 15-20 holds, with feet, (the gyms i have worked at have had 35ft walls and typically a route had 20-25 holds) in which case this would be enough. But you also want a variety of holds. If this is 20 horizontal, than you will want a few more, but more different types/sizes than just more quantity. I have worked for a few indoor gyms (East and Mountains) and climbed in 100s of competitions, to make a good boulder problem takes maybe 10-15 holds and just takes creativity and imagination...also calls for comfort. Do what comes naturally, learn to set and climb and then branch out. There is a good book from REI on Indoor Route Setting, you should get it. Oh and 1 hold per sq. ft. is good, if you can use it all, but i am assuming that you won't use most of the bottom and top corners or most of the top foot and bottom foot of the wall, so depending on orientation, that saves you some sq. ft.

First of all, you're going to need more holds than this for 140 square feet of wall. A good estimate is one hold per square foot at least, and this only has 96. And second, no one really has a structured system for setting up routes, so you're unlikely to find any tips on that. Being a good routesetter just takes practice. You have to set holds in positions and sequences that you think will be fun, and you'll learn pretty quickly what is and isn't.