There is an unlikely expression that can sum up any field/key in an array, and that's the XPATH expression. The XPATH allows you to sum up any level of the array, it does require the json array to be converted to xml, but we have the XML expression for that.
The whole process is 3 steps
The final expression converts the object into XML and then uses the XPATH expression.
xpath(xml(outputs('ComposeConvert')), 'sum(/root/number)')
Xpath can do Min,Max,Average,Count as well as Sum. It can also use And/Or operators to filter cross fields, and much more.
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