Office VLAN subnetting

Hello guys!

I have read the rules and I hope I don't break any of them with this post.

I'll try to make this as short as possible. But before I continue, just to give a small resume:

I am an IT engineer, but I wasn't in the field until recently. Basically, my master was networking, I graduated 2016-ish. Some courses we had were UNIX and cisco etc. Also took a MTCNA to see how that works, so I am working on refresh courses.

I started a new job position in a small town (was the only one who applied for this position) to manage a office building that has 16 offices where startup companies can rent a office for their business.
At my disposal, I have 3 Mikrotik 48port cloud switches and a Mikrotik Cloud core router (also waiting for ISP for fiber optics, so for now I have a hybrid box from T-com) so - each office cannot see another office on the network (due to GDPR, EU). This I know that I need VLAN's to make that happen.

Now, maybe I went too far with this post, but would I need to dabble in subnets for each vlan?
Should I just make each vlan with a x/24 or considering that an office has a max of 12 eth ports (if that's the correct terminology?)

Also, someone mentioned to me to acquire hardware firewalls (one for redundancy). Is that a mandatory option? What are your opinions on this?

I hate to be this guy, who asks for help online, considering all of you who are masters of this trade, worked very hard to get where you are today.
Hope you are all doing well and look forward for any help or guidance you can throw my way!