How can I obtain the number（2016） of ro-ro passenger ferries in the world？?

One way would be by doing an actual census – which might be achieved by writing to the Ministry of Transport (or equivalent government officer) for each coastal country and asking for either the specific number or for their advice on how to account for those vessels operating in their coastal waters (and registered to that nation, so you don’t double count, because you wouldn’t want to count ‘all ferries operating in Denmark’ and ‘all ferries operating in Sweden’ to find that you’re counting the same vessels twice).

Alternatively, you could review fairly high-level maps (that is, not the super-detailed navigation charts, but ‘overview’ charts or maps) of coastal areas to view the ferry routes, and then do research on the mainland and inter-island routes online to see what vessels are operated and get your numbers that way. That might seem to take longer, but it would probably be more accurate, and might be quicker in the long run because you could perform the assessment yourself (or you and a small, dedicated team) rather than wait for various Transport Ministers (or their deputies, or some flunky) to respond. Doing it yourself would also enable you to have more assurance in the quality of the data, too.

Perhaps one of the best ways to get good rough numbers quickly would be to contact the shipbuilders who make these vessels and ask them. (It might help if you are doing this as some kind of scholarly study and can provide evidence of your bona fides. They would probably be unwilling to share some information with competitors, but if you are scrupulous about handling confidential data, then they might open up to you.) They will necessarily have some pretty good experience with the as-is fleet (in terms of maintenace and repair records, boats expected to retire within the next “x” years, etc. And there are far fewer builders, I’m sure, than there are owners and operators and countries with ro-ro ferry lines.

I found a website which says that the number of ro-ro ships in the world as at Jan 2015 is 4066.
Another statistic on this site gives the number of passenger ships in the world merchant fleet as of January 1, 2016, by ship type

Oh, sure, @Stinley… go ahead and do it the easy way by just looking it up. Seriously, though, that looks like a good site to know. Thanks for that. (The OP’s number had seemed to me to be unreasonably low. But the category of “ro-ro ferry” is so broad, from two-car ferries across fairly narrow rivers in good weather to huge ocean-going vessels that operate in nearly all sea and weather conditions, that he may have been focused on only the largest and most robust vessels in the fleet.)

@CWOTUS I am a librarian not a researcher. We look things up; we don’t conduct primary research!