In a 6-node ONTAP 9.1 cluster, an HA (High Availability) pair consists of two nodes. If you want to count how many new FAS9000 HA pairs could be joined to this cluster, you need to consider that each HA pair adds 2 nodes. Given that there are two FAS6290 nodes and four FAS8080 EX nodes, there are already 6 nodes in the cluster. Therefore, the cluster is at its maximum node count, so no new FAS9000 HA pairs can be added without exceeding the cluster's capacity. Since the question asks about the formation of HA pairs within the existing 6-node cluster, two HA pairs can be formed (one pair from the two FAS6290 nodes and one pair from the four FAS8080 EX nodes). Thus, there will be 2 FAS9000 HA pairs.