How many days did the disciples wait in the upper room for the day of Pentecost?

We simply do not know.

We can assume that the twelve were there (the 11 original disciples and Matthias who replaced Judas Iscariot after his suicide), but in addition to these it is most likely that other followers of Jesus were there too. These would have included those who were present at his crucifixion (eg Mary, his mother and Mary Magdalene and Salome, as well as others) and probably other followers such as Cleopas and his friend (thetwo men the resurrected Jesus appeared to on the road to Emmaus), John Mark, Lazarus, Mary and Marha from Bethany, Joseph of Arimathea and possibly Nicodemus too. However as to actual numbers nowhere in scripture does it tell us this - only that 'all' the believers were assembled.

Actually there were just shy of 120.

13And when they were come in, they went up into an upper room, where abode both Peter, and James, and John, and Andrew, Philip, and Thomas, Bartholomew, and Matthew, James the son of Alphaeus, and Simon Zelotes, and Judas the brother of James.

14These all continued with one accord in prayer and supplication, with the women, and Mary the mother of Jesus, and with his brethren.

15And in those days Peter stood up in the midst of the disciples, and said, (the number of names together were about an hundred and twenty.