For now I will only use it on one site but I'm sure that within the next months I want to extend to some other sites I'm working on.
If you like, you could purchase the Single-Site license just now and then upgrade to the 5-Site license at a later date (for the difference in price between the two). Please see this FAQ:
Can I upgrade from a single site license to a multi-site license at a later time?
When I purchase the 5 site license, do I already have to specify on which domains I plan to use these?
No. There is no need to specify which domains you plan to upload your Juicebox-Pro galleries to. You can pick and choose your domains as you like. You could purchase a 5-Site license before you purchase your domains (or even decide on your domain names).
Can I move the license from one domain to another, so stop using it on one domain and start using it on the next?
Yes. You are free to change the domain(s) that you have your Juicebox-Pro galleries uploaded to at any time you like.
As long as you have all your Juicebox-Pro galleries on only one domain (for a Single-Site license) or on no more than 5 domains (for a 5-Site license) at any one time, that is fine.