Could the ISS hold five more folks for a couple months?
I don’t know. They will have some space by staying in the shuttle, and can go to iron rations for food. Power and water and oxy are the limiting factors, in my grossly uninformed opinion.
How many people up there now?
Short answer: Yes.
Somewhat longer answer: It doesn't need to.
Full answer: This isn't an Apollo 13 situation. The orbiter is not crippled. It is up and fully functional, and the crew can continue to live and work on Atlantis for at least the remaining nine days scheduled for the mission -- probably at least twice hat long, because there's some wiggle room built into the system.
If they ration food and water, they can stretch those supplies. If they conserve effort, they can cut their O2 consumption. And they have backup O2 systems. Bottom line, the ISS plus a shuttle is a luxury hotel compared to Mir, and no one died on Mir. They learned. And they have redundant backups.
In the worst case, which is unlikely, Atlantis cannot re-enter -- but its resources can be exhausted with little risk before it is ditched. Another Shuttle can be launched on a rescue mission on relatively short notice -- that's part of the post-Columbia plan.
Meanwhile, three crew members could go home on a Soyuz any time on a moment's notice -- they keep one docked for just that purpose. The Russians can send up a Progress capsule -- an unmanned version of the Soyuz -- on a short turn-around to carry supplies up and bring three astronauts at a time down. If need be, and again it's unlikely this will even become an issue, the station and shuttle could be safely abandoned in a week or two.