We currently have two zebrafinches, both male, however in recent months the oldest (who is atleast 3years old) has spent all its time on the cage floor and only flies up to perches when we put out hands in the cage to replace food and water. We are taking it to the vets but fear it may have to be put to sleep, this leaves the one (who is about 2years old), we are now wondering about getting 2/3 more to bring the number up to four if the other bird has to be put to sleep.
However my parents were thinking about not getting any more and leaving it until the last one dies to get a budgie, this creates the problem that im worried the bird may get lonely as the house in empty alot of the day with people at work and school, so im worried it could get depressed and this would be cruel, I wanted an opinion on whether it would be best to get more or if it would be ok to just have the one, if we interact with it as much as possible when we are in the house?

Hi, zebra finches definitely need some company. They cuddle up together at night and react together in the day. It would be cruel to keep just one on its own.