What is the best database to use

I use an Ubuntu 18.04 LTS Server and wanted to know the best database to use with Panda3d. I am going to convert a standalone game to multi-player. All almost all graphics, models etc. are to be stored on the local personal computer and only a few graphics possibly the composition codes for the other player’s avatar would be sent plus all of the of the data to control the movements of the models under server control would be sent, as well as the computational results of the players actions.

These concepts are not connected in any way, Panda3d and the database. You can do this as you like.

The database would hold current positions of computer controlled models / items. These would be sent to other players. It will additionally contain information about thins like inventory / possessions, character level, skill level, Item drop tables for successful fights just to name a few. The list of information stored on the server side could be quite lengthy and possibly in excess of 100,000. For example take another game WoW (Used for example only.) has almost if not more than 25,000 items that a character could conceivably come in possession of over the course of play. I am not saying that the game I am referring is anywhere as sophisticated as that. The database holds the applicable text/numerical variables the server side uses what it needs and passes some to the client side and also receives and stores some things from the client. Think of the database as a repository for the server and client application information.

Any decent modern database system (MongoDB, Postgres, whatever) can easily handle collections of upwards of 100k entries, which really is not very many by current standards. The choice of database should be part of your overall server architecture design. It is really not tied to Panda3D. So I think you are not asking the right questions at this point.