Thirty-three is 3 x 11. Its principal association is the age at which Jesus is conventionally said to have died, and that figure is an inference rather than a datum. No gospel states it.
The reasoning runs as follows. Luke 3:23 says that Jesus was about thirty years old when he began his ministry. John's gospel mentions three Passovers during that ministry, implying a span of a little over two years, which gives an age at death in the low thirties. The Synoptic gospels mention only one Passover and would imply a ministry of about a year. The number thirty-three is produced by combining Luke's starting age with John's chronology, and it became conventional early enough that it is treated as fact in devotional and artistic tradition.
The underlying dates are also uncertain. The crucifixion is usually placed in 30 or 33 CE, with the argument turning on which years Passover fell on the day the gospels require and on the tenure of Pilate, who governed Judaea from 26 to 36 CE. Both years have serious defenders and neither has carried the field.
What can be said plainly is that Jesus was executed under Pontius Pilate, that the ministry was short, and that thirty-three is a reasonable estimate presented with far more confidence than the evidence supports. It is a good example of a number that entered tradition as a calculation and hardened into a fact, which is worth watching for elsewhere, it happens more often than the numbers themselves suggest.