You're setting up your website and it consists of a few different apps set up across a few different sub-domains. So you have a root domain website (yourdomain.com) and say 2 sub-domain apps; app1.yourdomain.com and app2.yourdomain.com.

You want to install Google Tag Manager on all of your domains but there's just one problem; you're not sure if you should create a separate Google Tag Manager container for each sub-domain and your main domain, or if you should install just one Google Tag Manager container on all 3!?

I would just install one Google Tag Manager container on all 3 domains. There's no need to separate them and it's more of a headache to manage multiple GTM containers. It might even cause some issues with conversion tracking and attribution, depending on how your website is setup.

The bottom line is it's just extra work that serves no purpose so why do it?

The rule I use is, "one domain (no matter how many sub-domains it has) gets one GTM container."