How to use the same switch to control multiple bulbs at the same time?

You can add multiple devices at the same time. Make sure they are all breathing rapidly, then just add them the same way as adding one device. After the devices are connected, you can give each of them a different name. You can control them individually in the app, or you can group them together with the group function and control them as one group. 

If it is hard to make them breathing rapidly at the same time, you can install each of them in a different lamp and add them one by one in the app. Once a device is connected to your app, it will remember the WiFi name and password of the router in your house. Then you can unscrew it and install it in the place you needed. It will connect itself to the same WiFi automatically

You also can group multiple same model devices to one group and control them together as one group. At the device's control panel, click the three dots at the top right corner, then choose 'Create Group', it will list all the devices which are the same model as this one.You can choose the ones you want to group and give the group a name. Then you will find this group on your device list, you can control this group the same as one device. Please note that if you are using Amazon Alexa, it does not recognize the groups created in PlusMinus app, you need use Alexa's group function to group different devices in the Alexa app.