Unless your company has extensive operational needs, certified senders should only need one or two IP groups. Sending from a large number of IP addresses makes it appear as if you are trying to water down or avoid bad reputation metrics. Also, it is easier to maintain a good IP reputation with fewer IP addresses.
Therefore, certified senders cannot have more than three IP groups when they are accepted into the program. They also may not add so many new IP addresses after they are certified that they exceed this threshold.