The phrase "someone's number is up" means that the person's luck or success has run out and they are about to experience an unfavorable event, typically death. There are several synonyms that can be used in place of this phrase such as "time has come", "the end is near", "their fate is sealed", "their days are numbered", "doomed", and "on borrowed time". These alternatives can be used in various scenarios to provide a more varied vocabulary. However, it is important to use such phrases with tact and sensitivity, as they can have a heavy connotation and impact on the listener.
