Number of fires is rarely a good indicator of fire season strength because it is so dependent upon highly variable human and lightning ignitions. Additionally, fire suppression is able to extinguish 99% of all ignitions. Fire size, or area burned, is also problematic because many of the largest fires occur in very remote areas and are often grassland or shrubland fires that can scorch 50,000 or even 100,000 acres in a single windy day. While these fires definitely have negative impacts, killing cattle and rendering rangelands un-grazeable for several years, they don't usually cost as much per acre to suppress because they impact fewer homes and less infrastructure.
Forest fires and fires primarily burning in the Wildland Urban-Interface require the most resources to fight, making them the most expensive per acre in terms of suppression costs. These fires become first priority for suppression resources because lives and homes are at risk. When enough of these types of fires are burning with extreme fire behavior at the same time, a larger proportion of national firefighting resources are committed to trying to stop them. Nationally, PL4 and PL5 are primarily reached when multiple Geographic Areas (GACCs) have multiple large fires that threaten homes and critical infrastructure. Today, for example, there are 4 GACCs at PL5 for the individual GACCs (Northwest, Northern Rockies, Northern California, and Great Basin), and the first three have all been at PL5 much of the month. So the national PL level is 5 as well.
2002 holds the current record (since 1990) in terms of number of days at PL4 or 5, due to an early fire season with large incidents in both the Southwest and the Rocky Mountains (including the Hayman Fire in Colorado). For NIFC to break that record this year, fire season in the west would have to stay severe well into September, with little to no rain and above normal temperatures. It is possible, but let's hope that's not the case.