So that's the way to count all the guilds member in one message!
const math = require('mathjs');
const guildCount = client.guilds.cache.map(g => g.memberCount);
const reslut = math.evaluate(guildCount.join("+"));
message.channel.send(`Your Bot serve: \`${reslut}\` Member!`)
Well, your code adds the memberCountz of each guild to the calvariable, but you are using theguildCountvariable for the sum, which contains nothing since your.map` callback is not returning anything.
So either
const math = require('mathjs');
const guildCount = client.guilds.cache.map(g => {return g.memberCount});
const result = math.evaluate(guildCount.join("+"));
message.channel.send(`Your Bot serve: \`${result}\` Member!`)
or
const math = require('mathjs');
const cal = [];
const guildCount = client.guilds.cache.forEach(g => {cal.push(g.memberCount)});
const result = math.evaluate(cal.join("+"));
message.channel.send(`Your Bot serve: \`${result}\` Member!`)
But you can do the sum with a simple reduce, no need for an additional package.
const result = client.guilds.cache.reduce((total, guild) => sum + guild.memberCount, 0);
message.channel.send(`Your Bot serve: \`${result}\` Member!`)