Can I purchase license several times and add license as needed or later?

Yes, you can. There is, however, various Editions (how to tell) have their respectively notes, as follows:

For Commercial Edition (or Enterprise Edition): Yes.
For Lite Edition, please be informed there is a single PC 64-ch limit. While Commercial Edition or Enterprise Edition does not have this limit, there are still potential system performance bottleneck issues that need to be considered carefully.
You cannot add Standard License (which is for Lite Edition) to Commercial Edition or Enterprise Edition. Similarly, you cannot add Professional/Intelligent/Panoramic License to Lite Edition.
For Community Edition, please check this article: How to extend NVR Community Edition function through Patreon platform?
For Subscription Edition, license can always be added later. Please note online licensing and subscription license cannot be mixed.

Please also note:

If you add license to a running system, it can exceed the system performance capacity, therefore causing unwanted result. Please read the FAQ: System integration is a specialized expertise.
Support request over systems that have operation history over a year is subject to paid consultant service. If you add license to such server, you might not qualify for free tech support. See Notice to System Integrators.