Hey Tech YouTuber & 8 Year Minecraft Veteran here,
First off you will need 2 cores at least to run the OS + 8 - 16 (With simultaneous multi threading) cores for the server. As for FPS i'm confused on what you are asking. I am going to assume you mean that you want to play simultaneously as you run the server. You will need at least 64 GB of ram to deal with all of those players and for you to run your game. A 1050ti would be fine because neither the Minecraft client or the server itself uses the GPU heavily.
Also the way Mojang coded the server you really can't get past 400 people on the same world without the resource requirement becoming too great. 400 people alone would require those specs. If you wanted multiple worlds that would be easier to accommodate. If you wanted that all to be on one machine I would recommend an AMD EPYC CPU perhaps a 2 socket system. The final price for something you requested would be around 10K USD. That is if you broke the players up into separate worlds.
My credentials: (Discussing minecraft client side hardware requirements)