try this you can use an alias for COUNT(*)
<?php        
    $connection=mysqli_connect("host", "username", "password", "database");
    $sql="SELECT COUNT(*) as cnt from database_users";
    $res = mysqli_query($connection, $sql);
    $users = $result->fetch_assoc(); 
    if ($users['cnt'] < 4) {
?>
    // html goes here, outside of the php tags
<?php
    } else {
        echo "Sorry, you have reached the user account limit.";
    }
?>
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solved Limit registration to 4 people on form sign up [duplicate]