A human without Exceptional Attribute in either of the initiative affecting attributes would be able to have a maximum Initiative attribute of 20 ((6+4) Reaction + (6+4) Intuition) assuming they had 6s in both Reaction and Intuition already. They could get that through sustaining Increase [Reaction] w/ 4 net hits and Increase [Intuition] w/4 net hits. They could alternately do it by getting 8 net hits on Increase Reflexes.
Since the initiative attribute is derived, I don't really consider it being capped as the others, but capped based on the attributes used in the derivation.
For a further example, let's say a runner has Reaction (4), Intuition (4). His maximum Initiative attribute is 16. He could get that by having a mage cast Increase Reaction w/2 net hits, Increased Intuition w/2 net hits and Increased Reflexes with 4 net hits (or any combination thereof that adds up to
. Not sure if that is clearer or not...