I want to group 10 HomePods as a single speaker for whole home audio.

You can AirPlay to all 10 but cannot set them up as one group of 10 in the home app. You can only pair two of the same model that will always act as one, when chosen. I will tell you that with this many one or two are likely to drop out. I have 9 and one or two might fail to play. Using Automations, this is less likely to happen. I suggest starting with that. You might need an Apple Music subscription or iTunes Match to choose audio in this within the Home App.

What’s required before you begin:

• All HomePods should be running the same latest software version
• All should be signed into the same Apple ID / iCloud account - this includes the devices you’re using them with
• All should be using the same home wifi network - this includes the devices you’re using them with

You have several ways you can get them to play to all:

• By asking Siri “play everywhere”
• Manually selecting each one on the device you wish to AirPlay from
• Creating Automations where you select them all to be used.

I should add some links for you to use here but my iPad is acting up trying to source pages. You get the idea, hopefully.

You’re welcome.

Apple welcomes all feedback. Not saying they will do it but you can at least inform them of your user experience: Feedback - HomePod - Apple

some troubleshooting steps you can take to resolve issues:

• rebooting the wifi
• Restarting HomePods - do this from the home app or by unplugging for 10 seconds.

it’s my observation that HomePods will always play together how they were last used. So if you used all 10 and it worked, once you played one or more alone, they become unlinked again. Or it can be one disconnected from wifi at the very moment you attempted to play them . Or there’s a slight delay in one of them receiving the request to play. For myself I do prefer the Automation method. Music begins each day at a set time to various HomePods in different rooms. If I feel like listening to this again on the chosen HomePods, I go into this Automation and use “test automation”. Now there might be better ways by using Shortcuts. I’m not experienced with shortcuts to explain how but you might give it a try to see if you prefer it.