Hi Isamael,
unfortunately I have to change the theme though we have a Campaign and had to have the shop work. I tried with some other themes and they all work with Mix and Match.
If you Google “WO Mix and Match does not Count” you can read the following on the plugin support page:

When using Mix and Match, you need to make sure that your theme is using the WC()->cart->get_cart_contents_count() method to count cart items. If you don’t know how to verify this, it might be a good idea to share this information with your theme author(s). Note that by default, Mix and Match contents are not counted. A Mix and Match assortment will be counted as one item, no matter how many products it contains.

Yes it worked BUT the images is blurred/unsharp.