Does Adding Subdomain Take Up My Systeme’s Custom Domains Quota?

Yes, domain and subdomain are treated the same. When you add a subdomain to your Systeme account, it will deduct your custom domain allowed quota. For example, the Startup Plan for Systeme io allowed three custom domains. Assuming you add 2 subdomains and 1 domain, that will max out the given quota in the Startup Plan.