What is the max per month for usage fees ?

For example, if John Doe owns 5 sims, does he get charged 5 times 195 dollars per month ? Or is it capped ? The website is not explicit on this.

Yup, you're on the right track, steve.

Above ownership of a full region (US$195.00/month as it says on the Land Fees page), tier increases in 1/2 region increments, with each increment being US$97.50/month.

So, if you own a region-and-a-half, 98304 m2, that's US$292.50/month in Land Fees.

Five regions? US$975.00/month, yes.

Also, good to remember that private island billing is different from that on the mainland, and the Concierge should be contacted for your account specifics. Take care!