Can we still follow same publishing structure in Sites 9?
Yes, take a look at the Sites 9 legacy documentation, here.
However, you should really consider migrating to the new architecture as soon as possible.
And how much efforts it will take to upgrade from 8.5 to 9 (considering we only have 15-16 Sites hosted in CMS)?
That question is impossible to answer remotely because of the lack of information ... how big is your implementation, how much content is there, do you use any additional SDL modules, etc. etc.?
Having that said, it's up to you to go through the current setup and make an analysis + estimation for all parts of the system, including the architecture, code (API changes), and all additional activities (like possible republishing, testing, etc.). Only then can you estimate the total effort needed to upgrade the entire system.
Going from 8.5 to 9 (and onward (9 -> 9.1, etc.)), there is the option of doing staged and rolling upgrades which significantly reduces an upgrade's complexity, however I believe it only applies to microservices based architecture. I couldn't find a definitive confirmation on this, but at no point was legacy setup mentioned as being compatible. Another reason to upgrade the architecture :).