Guild members without bots always 1

Fetch all the GuildMember's first and access the returning collection, then sweep (remove) all the members who are bots.

let real_members;
interaction.guild.members.fetch().then(all_members => {
   real_members = all_members.sweep(member => member.user.bot);
});

console.log(real_members.size);

Ensure you have Intents.FLAGS.GUILD_MEMBERS included in your client's intents aswell as enabled in your Discord developer portal

Discord.JS Collection Utility Functions

Maybe try using ".fetch" so:

interaction.guild.members.fetch()
.then ((members) => {
    const AmountOfMembers = members.filter((member) => !member.user.bot).size
}) 

CODE IS NOT TESTED

To be able to access it outside either define variable outside or turn it into a function and return the value.

function getMembers (interaction) {
    interaction.guild.members.fetch()
    .then ((members) => {
        members.filter((member) => !member.user.bot).size
        .then((Amount) => {
        return Amount
        })
    }) 
};


const AmountOfMembers = getMembers(interaction);

Just pass it your interaction.