Mount up is an idiom that means to increase or accumulate. There are several synonyms for this phrase such as escalate, rise, grow, develop, surge, amplify, and augment. These words can be used to describe situations where something is getting bigger, stronger, or more intense. For instance, you might say that the tensions between two countries are escalating, or that the number of coronavirus cases is surging. If you want to express a gradual increase, you might choose words like grow or develop. If you're talking about something that's becoming more forceful or powerful, you could use words like amplify or augment. Overall, there are many different synonyms for mount up that can be used depending on the situation.