You really can never have enough cables. Get all lengths available and as many of each as you can if each - you never want to be in the situation where you run out.
These come individually bagged so be prepared for a morning spent unbagging! I've ordered hundreds of doepfer cables and not had a single dud (which I can't say for other manufacturers). Any kinks in the cables can be gently ironed out by hanging over, or next to, a warm (not hot) radiator over night.
When plugged in:
A-100C80's will happily reach corner to corner on a Doepfer LC9 (9u x 84hp) system.
A-100C50's will almost reach corner to corner on a Doepfer LC6 (6u x 84hp) system, and reaches corner to corner on a LC3 (3u x 84hp) happily.
A-100C30's are great for passing signals from a module to a module in the row below, as reaches from very top of the module to very bottom of the row below when plugged in.
A-100C15's are wonderful for passing signal to the next module along in a row, and are preferable for keeping the workspace tidy of hanging patch cables. I also use these ones for dummy plugs for breaking normalled connections as they don't hang down too far and get knocked (thereby generating unwanted signal).