1You need to use client.setActivity() and then use the client.guilds.cache.size value. 
2For example:
3
4client.setActivity(`Currently in ${client.guilds.cache.size} servers`);

1module.exports.run = async (client, message, arguments) => {
2   const guild = client.guilds.get("566596189827629066");
3   setInterval(function () {
4      var memberCount = guild.members.filter(member => !member.user.bot).size;  
5      var memberCountChannel = client.channels.get("626462657817477131");
6      memberCountChannel.setName(`${guild.name} has ${memberCount} members!`);
7   }, 1000);
8};