I'm not sure about how many waypoints Litchi supports in a single mission upload. The limit of 99 waypoints while using GSP is tied to the fact that the waypoints are stored in the aircraft's internal memory (not the SD Card). The GSP will subdivide the total waypoints for a mission (can be hundreds) into 99 waypoint chunks. It loads the first 99 and then fly’s that chunk. Then it allows you to load the next 99 waypoints and continue flying. This process repeats until all waypoints have been achieved. For Litchi, you can certainly create missions that have hundreds of waypoints but it may handle them differently. GSP considers every photograph as a waypoint (in Hover and Shoot mode) which is why seemingly simple missions can be hundreds of waypoints. Litchi may do this differently. Sorry I can't be of more help...
As an aside, “Why does DJI use internal memory?” Several reasons: 1. The Mavic’s internal memory is much faster than the SD card so there’s less latency in data access. 2. The Mavic’s internal memory is also used for all of the Aircraft’s logging data, configurations, mission parameters, telemetry data and, of course, waypoint data. 3. By using the SD card only for imagery data, SD cards can be swapped between missions without loosing critical configuration data. And Lastly – 4. By using internal memory, DJI controls the quality and speed of critical memory components which is a form of failsafe/reliability control. Especially if/when there is a loss of controller link. The Aircraft still has everything it needs to safely perform it’s emergency procedures.
Why do I not use Litchi? There are many people using Litchi and love it. I prefer to use GSP and Go4 over Litchi because
a) I haven’t found a single mission scenario where I couldn’t achieve it using Go4 or GSP.
b) I believe GSP/Go4 are more stable software than Litchi and
c) I have three colleagues who have had issues with their Mavic going haywire while using Litchi. Again, just my experience and preference. There are just as many pilots out there who prefer and use Litchi.