1.5 ton needs 600 cfm to work properly. Assuming you have 6” flex and 4x10 or so vents that’s roughly 100 cfm per vent. This is very rule of thumbish as those number can vary a bit depending on many factors If you still have at least 6 vents remaining it should be ok but I would recommend leaving it as your still working with the same amount of sq footage. That room might suffer a bit with only one vent. You could also upsize the remaining vent to compensate.

What everyone seems to be missing here is it isn’t about how many vents, it’s about how many cfm. 1 vent could be 100cfm another could be more or less. Before you add or remove ductwork you need to know the requirements of your particular house and hvac system . A property installed system has a balance that is critical for functionality and efficiency Find a good hvac contractor that can do this math before you remove it.