The spacing between balusters depends on many factors. Besides the ornamental role that these balusters have, they also provide structural support for the handrails and also security for children and pets from slipping through.
Usually balusters don’t offer s great support for the handrails.
There may be some regulation that provide the needed spacing between the balusters. Some say that they need to be 4 inches apart, but this depends a lot on the thickness or width of the balusters. It also depends on the design. 6 inch balusters cannot be installed with a 2 or 4 inches spacing between them. The spacing needs to be small enough to avoid children and pets from slipping through them.