Hello, Cristina,
1/ It is because the Ajax add to cart option was disabled, to make the adding to can work even when the YITH WooCommerce Product Add-ons & Extra Options Premium plugin is activated.
You can enable the Ajax add to cart option via Theme Options WooComerce > Single product page > Diseño de un solo producto, if you decide to disable the WooCommerce Product Add-ons plugin or in case the authors of the plugin find another solution.
2/ Go to Theme options > Header Builder > Cart > Número De Productos En Mini-Carrito change 3 to the desired value.
Kind Regards,
8theme team