a simple reasearch on singapore airlines website will get you this.

from krisflyer star alliance award conditions

l. One origin or destination open-jaw is permitted for a round-trip award (e.g. fly Singapore to San Francisco, then LA to Singapore; or fly Seoul to Singapore, then take a return leg to Tokyo). Open-jaws are permitted across different award zones but the higher award level will apply. En route open-jaws are not permitted.

f. One complimentary en route stopover is permitted for round-trip awards, unless otherwise stated below. Up to three additional stopovers are also permitted at USD100 each, regardless of the class of service.