If it’s you’r first project, don’t go for 100 player : this is an absolute fail and require a lot of network experience.
Aim for something a bit lower.
But to answer to you’r ask, unreal can handle 100 players (or even more), all depend the optimisation you gonna do.
There is some built in stuff like replication graph, and network optimisation you will find in the documentation.
I don’t think blueprint will allow such optimisation and this kind of thing require c++ to have a good control over how network stuff work in your game.