Unless you are running BC version 13 or older you will not need a developer license. They are obsolete for newer versions.
What you need to purchase, if you run BC on prem, is access to objects like tables, reports, pages and code units and they are added to your ordinary license.
If you are running BC online as a subscription service all the objects are already included in your license and you can start development in AL with out any additional license.
You should user the object range between 50 000 and 99 999.
This range is for customizations, and for test purposes. For Business Central online, a partner can develop an extension tailored to the individual tenant to fit the needs. The partner will develop this either by using a sandbox tenant or by obtaining a Docker image. Once the development is done, the extension can be deployed to the individual tenant.
Also, use this range as part of training and similar, such as if you are using a sandbox tenant or a build of Business Central on Docker.
So in BC online you can start developing with out any additional license as long as you have the level of AL knowledge that is needed.
