I did a quick search.
The 2012 Olympics had 10,768 athletes. That doesn’t count the coaches, trainers, staff, families, visitors, or anyone else. That’s only the athletes.
In the 2012 Presidential election, the 2 major parties had their conventions.
D 5554 delegates attended
R 2286 delegates attended
The Paul VI Audience Hall at the Vatican (which was built after Vatican II) can seat 6,300 people at one time.
Keep in mind that the bishops do not have to be all in the same room for every single moment of the Council. Even at Vatican II, they only came together as a whole for (relatively) brief periods.
In 2013, an average of over 51,000 people attended the outdoor papal audiences. That’s 51,000 people at one time. If we assume that just 10% of them were tourists (which is probably very low indeed) then that means that the city of Rome can easily handle 5,000 visitors on any given day.
magister.blogautore.espresso.repubblica.it/2015/08/27/anche-in-vaticano-e-recessione-i-numeri-della-prefettura/
The city of Rome itself has over 44,000 hotel rooms and over 87,000 beds (some bishops might actually want to have a roommate).
scribd.com/doc/17890728/Presenze-Turisti-Roma-marzo-2009
That is only hotels within the city limits. It doesn’t count those nearby the city.
Finding a place to accommodate about 5,000 Catholic bishops doesn’t seem too difficult.
There’s no doubt that a ecumenical council would require a lot of logistics—none whatsoever.
Still, I don’t think that an ecumenical council, attended by 5,100 bishops would be beyond the capacity of Rome.