Another thing to consider with regard to multiple familiars is classes or class features which require a familiar of a specific kind. For example, a Druid with the Eagle Domain gains a Hawk familiar ("You gain a hawk familiar. Your effective wizard level for this ability is equal to your druid level. Your druid level stacks with levels from other classes that grant familiars when determining the powers of your familiar"), while a Bard with the Sea Singer archetype gains a familiar that must be a Parrot or Monkey ("At 2nd level, a sea singer acquires an exotic pet—a monkey or parrot (treat as raven)—that gains abilities as a wizard’s familiar, using the sea singer’s bard level as his wizard level.")
By the "you may only have one familiar" interpretation of the rules, what happens when an Eagle Domain Druid multi-classes as a Sea Singer Bard?
Or, since Clerics may take Animal and Terrain Domains, what happens when a Cleric takes both the Eagle (Hawk familiar) and Serpent (Viper familiar) domains at 1st level? Clearly said Cleric is able to have more than one Familiar.
Thus characters must be able to have more than one familiar...
So it stands to reason that ojamojallo reading of the rules is correct. If a class grants a familiar, you gain a new familiar AND your levels in the relevant classes stack for determining their abilities.