I can't add more than 31 selling plans to a subscription rule, why is that?

There is a specific limitation set in Shopify, which prevents us from adding more than 31 selling plans per subscription rule. When our system tries to add more selling plans, Shopify API responds with an error message: "Selling plan groups can't have more than 31 selling plans." 
Unfortunately, this limitation is set on Shopify's side, so we can't bypass it, but when Shopify increases this limit, we will immediately update our app to allow you to add more selling plans to the subscription rule.