Harry Potter and The Half Blood Prince - 3 Tables or 4
After Ron gets out of the hospital wing, and is sitting down and talking with Harry and Hermione we see that there is 1 table behind Harrry and 1 behind Ron and Hermione. That would make a total of 3 tables. But when Harry sstands up to talk to Katie Bell we can clearly see the 4 house tables.
Actually if you look carefully, you can see Lavender Brown sitting on the far right table [looking into the hall from the double doors] There is a table in between her and the table Harry, Ron and Hermione are sitting on... for some odd reason nobody is sat at the table, but it is there