The Licensing and OEM sections of our FAQ really tell the whole story. See here.
In short, they say that if your are using Express Studio you will need to keep the LGPL waiver visible. If you want to get rid of it, you need to purchase a Professional studio and/or OEM/SaaS license.
If you run your applications for your own organization, or users of your own organization (no limit for number of users), Express Studio (if you are ok with the waiver) or Professional Studio license is sufficient. Under this license, you can have as many applications as you want, as long as they are run for your organization.
If you start to distribute your application in any way, either by selling your application to other organizations, or you offer your application as SaaS for other organizations, then you can have up to 5 such agreements under the Professional or Express license. This counter counts for individual named applications, so if you have two different applications, selling each one to 5 other organizations, you would be covered by this license. When you need the 6th, you would need OEM/SaaS license. The same counter would be used for application you create for another organization which then resells your application to others. Distributed to Max 5 organizations in total for each named application before you need OEM/SaaS.
Hope this clarifies,