Since I don't work for them, this is not a official Stardock answer, but it is a reliable one.
1. is there an activation limit?
Yes to a degree. If you hit your limit, support will reset it for you.
2. do you have to pay each time it gets an update?
No, if you buy the stand alone version of Windowblinds, you get free updates until the next number version release. It's currently at 7, the next version release will be 8.
3. will I be able to install it on 2 maybe 3 computers? (Main computer and laptop) without the original computer or other computers giving me activation errors?
You are allowed to install on two machines as long as you are not using both at the same time and they are owned by you.
4. do people still make themes for it? (I remember using it in the XP days and loving it)
Yes, there are new additions to the gallery all the time.