If you mean you just want one alert and all others to silently add to that one alert icon, Arlo doesn't do that. It may be better to figure out a way to minimize the number of recordings (hence, alerts) in the first place by repositioning the camera and/or changing the motion sensitivity in your modes and rules.
Since you mention you can't find where to create a mode, read the FAQs here on this and many other topics which give you step by step procedures. Basically, use the Mode tab, click on your hub and check your modes. You can also create your own custom modes while in there.