Many building codes stipulate that a 4" sphere should not be able to pass between any space in the balustrade. That would, of course, include spaces between Balusters.
The following are the maximum widths between each pair of Sawn Balusters when they are installed touching side-by-side:
SB#6 - 3-5/8"
SB#5 - 2"
Classical Urn - 4-3/4"
Spear - 4-1/8"
Heart & Fleur - 3-3/8"
Sawn Balusters can alternate with plain pickets to help meet the 4" sphere code requirement.