Hi @danielpekel03,
In WooCommerce inventory for variable products can be managed on product or variation level. To manage stock on variation level, you need to enable the option “Manage stock?” for each variation and then set a stock quantity. Alternatively, you can toggle “Manage stock?” on and off for all variations or set the same stock quantity at once using the corresponding options on the select box on the top of the “Variations” tab when editing the product.
I believe you will be able to achieve what your client needs if you manage stock on the variation level.
Here is a screenshot showing a variation product that has “Manage stock?” enabled:

And here is a screenshot showing the option that lets you enable stock management for all variations at once:

Thank you for your answer, but it’s not exactly what I’m looking for. Because my client has products with at least 4 variations and what he asks from me is to make 2 groups of variation. So if my client buys one fence of a specific size and a specific wall mount, the stock of the fence with the same size but a different wall mount also goes -1
Using the example above, there are three variations of the T-shirt. One for each color. If instead, we create one variation per combination of color and size, we will get what I believe is what your client is looking for.
In this new scenario, we would have nine different variations, and if a customer would buy a T-Shirt that is blue and large, the stock would be reduced only for this specific variation and not for all blue T-shirts:

It´s not exactly what I´m looking for because it doesn´t change the stock of the product with the same first variable.
for example:
“the main product name”
-lenght: 3.0m -mounting: attached to a wall
-lenght: 3.0m -mounting: pole, size 7*7*7
-lenght: 3.0m -mounting: pole, size 10*10*10
-lenght: 4.0m -mounting: attached to a wall
-lenght: 4.0m -mounting: pole, size 7*7*7
-lenght: 4.0m -mounting: pole, size 10*10*10
so if the costumer buys one of the fences, every stock of a fence with the same length has to go -1, and the costumer needs to see it on the front-end so they know that a specific fence size is out of stock.