The multiple argument is only used in the select controls.

Its value is an integer and defines the number of simultaneous options the user will be able to choose for the select control in question.

Defaults to 1.

Kirki::add_field( 'my_config', [
	'type'        => 'select',
	'settings'    => 'my_setting',
	'label'       => esc_html__( 'This is the label', 'kirki' ),
	'section'     => 'my_section',
	'default'     => [ 'option-1' ],
	'priority'    => 10,
	'multiple'    => 999,
	'choices'     => [
		'option-1' => esc_html__( 'Option 1', 'kirki' ),
		'option-2' => esc_html__( 'Option 2', 'kirki' ),
		'option-3' => esc_html__( 'Option 3', 'kirki' ),
		'option-4' => esc_html__( 'Option 4', 'kirki' ),
	],
] );