If you post this in the Conifer forum you will get a variety of very experienced opinions.
You did not specify a cultivar so I have to say that I do think keeping them to 12 feet may prove problematical unless you have chosen something really dwarf.

how to plant at link

yes they are conifers...

they really are going to be a lot of work to keep at 12 feet ...

spacing for screening.. is all about how fast you want a screen .. versus how long you are willing to wait for them to fill in ...

google the common name you gave.. find the latin name.. then google that.. then flip to the IMAGES side.. to see the potential for size ...

if there is an actual cultivar name .. the part in quotes.. it is very important to add that to any searches ...

ken

Here is a link that might be useful: link

Thank you for your responses. The cultivar name is Thuja occidentalis
(Eastern White Cedar) - Cupressaceae (Cypress Family)

I found out from the nursery that sells the trees that they should be planted every 50 cm (1.5 feet)

"I found out from the nursery that sells the trees that they should be planted every 50 cm (1.5 feet)"

Unless they are specific cultivar (you list the species) 1.5 feet would have them trunk to trunk in time. Are you planting a hedge or a stockade?

Here is a link that might be useful: Thuja occ

I planted several Thuja occidentalis "Emerald Green Arborvitae" against the south wall of my house years ago. My plants were small (about 12 inches). I spaced them about 2 1/2 to 3 feet apart. They're now about 8 feet tall and have filled in just right. Early on a couple of them didn't establish themselves and died even though they were well watered and in good starting condition. When I replaced them I dug plenty of peat into the soil and added a couple of handfulls of sulphur granuals. (My soil is heavy clay and neutral to slightly alkaline.) The replacements did very well.