As shown below, Discord only allows for 50 custom emojis per server (unless you’re a Nitro member) and the upload requirements are as follows: Accepted file types: JPEG, PNG, GIF.

If you want more emoji slots, you'll need to have users with Discord Nitro accounts on the server to "boost'' the available slots. If enough users boost your server, you can add up to 250 custom ...

Discord’s subscription-based Nitro service enables you to use emojis from one server on any other. I love dropping a random Tom Nook emote in a server that never talks about Animal Crossing.