you can also use a Jsonata with a regular expression to pick the number out of the string
$match(payload.alexaResponse, /\d\d?/)
Test Flow
[{"id":"6a18ea68e6e856fd","type":"inject","z":"54efb553244c241f","name":"alexaResponse 23","props":[{"p":"payload.alexaResponse","v":"Right now, it's 23 degrees Celsius","vt":"str"},{"p":"topic","vt":"str"}],"repeat":"","crontab":"","once":false,"onceDelay":0.1,"topic":"","x":190,"y":3900,"wires":[["8998cf9da4e6531c"]]},{"id":"8998cf9da4e6531c","type":"change","z":"54efb553244c241f","name":"","rules":[{"t":"set","p":"payload","pt":"msg","to":"$match(payload.alexaResponse, /\\d\\d?/)","tot":"jsonata"},{"t":"set","p":"payload","pt":"msg","to":"$number(payload.match)","tot":"jsonata"}],"action":"","property":"","from":"","to":"","reg":false,"x":420,"y":3940,"wires":[["b23ac979c073ab52"]]},{"id":"b23ac979c073ab52","type":"debug","z":"54efb553244c241f","name":"debug 62","active":true,"tosidebar":true,"console":false,"tostatus":false,"complete":"false","statusVal":"","statusType":"auto","x":620,"y":3940,"wires":[]},{"id":"676c3c1aab72c28f","type":"inject","z":"54efb553244c241f","name":"alexaResponse 9","props":[{"p":"payload.alexaResponse","v":"Right now, it's 9 degrees Celsius","vt":"str"},{"p":"topic","vt":"str"}],"repeat":"","crontab":"","once":false,"onceDelay":0.1,"topic":"","x":200,"y":3980,"wires":[["8998cf9da4e6531c"]]}]