Increasing the effects is not an easy to do simple task that someone could just give you instructions on how to do it and you go an do it without a problem.
In fact, increasing the effects limit is
not something that can
only be done on the server side either, its something that absolutely has to be done to the client as well, or else you will simply cause the clients to get the "debug" error and crash.
With all that in mind, the way you change the limit is by changing the underlying type to the enum. For the magic effects they are currently of type uint8_t, which allows 255 animations, to increase this limit you would change
C++:
enum MagicEffectClasses : uint8_t {
to
C++:
enum MagicEffectClasses : uint16_t {
That will provide you with a limit of this size: 65535
After you do that, you will have to find ALL instances in which MagicEffectClass is used, even if they don't declare that type explicitly (like they use auto instead), you must still find all usages of that datatype and change where they are used to properly handle and accept the new type.
Like I said above, once you have done that, you now must go over to the client and do the same thing, find the enum/datatype that represents the magic effects, change it's data type to the new one like above (uint16_t) and again, find all usages and make the necessary adjustments with the logic that uses and handles those data-structures to adhere to the new type.
Once you have finally completed ALL of those things, then you will want to fire up the server and test it with the client you made the adjustments to, specifically trying out the new effects (if you added any) or just effects in general to make sure nothing unexpected happens and that there is no crashing of the clients or server.