Normally airliners stay quite a distance from each other. Getting more than one in a shot is the result of compression of the distance as they pass in different directions. What is more fun is to have each jet be replaced by two. SFO likes to have the parallel arrivals and similarly the departures are often involving two planes at a time. If you time it right and have the angles aligned properly, you can get four jets in one shot – two on approach and two taking off.