"I would think there would be more."
More than 362 individual authors? Most people, when they come across the site, fail to notice the little check box at the top right corner of the page (under the name of the site), so it looks substantially less. If you did see that and wanted more, keep in mind, as brianpck mentioned, this doesn't include inscriptions, coins, etc.
That said, I'm sure one or two have slipped by here or there. Just checking over Cornell's Fragments of the Roman Historians, I don't see Tuditanus mentioned, although possibly this is due to the uncertainty surrounding the author. Vennonius too is absent, but with a single Latin fragment (also bound up with uncertainties), there are good reasons why he might have been overlooked. Same with Paulus Clodius.
PHI is a respectable database (and has been for decades), so yeah, there may be some omissions, but it is fairly complete with the caveat that this is for literary Latin.