Issue with Nav menu display

Hi everyone,

I have an issue with my nav menu.
On the menu i did put different catégories like this exemple :

category 1 | Category 2 | Category 3 | …

But after 5 or 6 category it stops and it continues under category 1, 2 … like this example :

category 1 | Category 2 | Category 3 | … | Category 6 |
Category 7 | Category 8

But i would like this :

category 1 | Category 2 | Category 3 | …| Category 6 | Category 7 | Category 8

How or what can i change to fix this please ?