Using wp_enqueue_style() function we can add multiple CSS files.

Syntax

wp_enqueue_style( string $handle, 
  string $src = '', 
  string[] $deps = array(), 
  string|bool|null $ver = false, 
  string $media = 'all' 
)

Parameters

$handle

(string) (Required) Name of the stylesheet. Should be unique.

$src
(string) (Optional) Full URL of the stylesheet, or path of the stylesheet relative to the WordPress root directory.

Default value: ''

$deps
(string[]) (Optional) An array of registered stylesheet handles this stylesheet depends on.

Default value: array()

$ver
(string|bool|null) (Optional) String specifying stylesheet version number, if it has one, which is added to the URL as a query string for cache busting purposes.

If version is set to false, a version number is automatically added equal to current installed WordPress version. If set to null, no version is added.

Default value: false

$media
(string) (Optional) The media for which this stylesheet has been defined. Accepts media types like 'all', 'print' and 'screen', or media queries like '(orientation: portrait)' and '(max-width: 640px)'.

Default value: 'all'

Proper way to styles

function namespace_theme_stylesheets() {
    wp_enqueue_style( 'mamies-wafers-bootstrap-min',  get_template_directory_uri() .'/css/bootstrap.min.css', array(), null, 'all' );
    wp_enqueue_style( 'mamies-wafers-carousel',  get_template_directory_uri() .'/css/carousel.css', array(), null, 'all' );
    wp_enqueue_style( 'mamies-wafers-style', get_stylesheet_uri(), '', null, 'all' );
}
add_action( 'wp_enqueue_scripts', 'namespace_theme_stylesheets' );

Credit: WP Dev