yes you can use a 120 gbs ssd to install windows, but you will not have space for you documents, desktop, well, anything really, windows uses like 30 gbs, any real good game uses 90 gbs, so it gets filled inmediatelly
i recomend you to buy a 240gbs minimum ssd
i have a couple pcs in a ssd 120gb in size, but i moved the users folder to a hard disk, together with steam, ubisoft, epic, origin games libraries, this way the ssd is only for windows
if you know how to do that, yes, the 120gbs ssd is enough as a boot drive, if not, try to buy a 240 or 480 gbs ssd as boot drive to avoid problems in the future