show amount of charge on apple watch 9

You can install a battery widget on your iPhone, or press the Side button on your watch to bring up the control center, or include a battery complication in your watch face.

Have a look at this related article and click on the blue links for detailed steps:

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• On some watch faces, you can add a battery complication. 
• On your iPhone, you can add a battery widget to show any connected Apple Watch. 
• When your Apple Watch is charging in Nightstand mode, tap the charging button to check the battery percentage.

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copied from: Check your battery and charge your Apple Watch - Apple Support

A quick press and release of this side button:

You can also say: “Hey Siri, battery”