When it comes to third-party systems, you can theoretically connect as many as you’re willing to pay for. As long as you are going through the API, you can add as many systems as you need for things like CRM, ERP, shipping, promotions or rewards, search solutions, or marketplace extensions.

Modules, however, are different than systems. Because they are made in a vacuum with no regard for other modules, best practices suggest limiting the number of modules to 10 or 15 maximum. If each of those modules affect a different aspect of your site, you should be fine. If you have more than one module for a specific function, such as promotions, you start running into problems. Your best option for incorporating modules is to leverage a customization through a certified partner.