To change the number columns in the cathegory page

Hi,

I have 2 questions :

1 – how can I change the number columns in cathegory page ?
I would like put just 1 column or 2. But not 3.(by default it s 3)
Is it possible to change that ?

https://www.skiescollection.com/?product_cat=domes-overcast

2 – I have a little problem with some thumbnails products from the cathegory page.
My page (domes overcast) have some blank (space) between 2 thumbnails products.
How can I fix that please ?

Let me know if you can help me please.

Thank you again for your help.