Up to 4 tentacle attacks when amorphous
The half-farspawn supernatural ability amorphous is like the special ability change shape that, in part, says that when the creature uses it "[t]he creature loses the natural weapons… of its original form" (Monster Manual 306). However, in addition to other changes to the change shape ability, the half-farspawn supernatural ability amorphous, in part, says, "The creature retains the tentacle attacks granted by this template… and gains two additional tentacle attacks when making a full attack" (153). So when a half-farspawn (Lords of Madness 151–3) half-illithid (Underdark 89–91) voidmind (Monster Manual III 187—9) commoner 1 uses the half-farspawn special ability amorphous, until it resumes its original form, it can take the full attack action to make up to 4 tentacle attacks, each at its normal attack bonus and with its normal Str modifier.
Changing form in this way (ahem) voids the extra natural attacks the templates have granted. That is, when the half-farspawn half-illithid voidmind commoner 1 changes shape into the amorphous tentacled mass, it isn't changing shape into a half-farspawn half-illithid voidmind amorphous tentacled mass—essentially double-dipping on its templates—but, instead, applying the effects of that change shape ability to the overall character broadly.
The half-farspawn half-illithid voidmind commoner 1 when it has changed shape into an amorphous tentacled mass just doesn't have in that form any more tentacles than the description of the special ability amorphous gives it. Even the "psionic" tentacle granted by the template voidmind is lost while the creature's amorphous—that sentient tentacle, too, is a natural weapon rather than, for instance, a special ability that allows the voidmind creature to create a psionic energy tentacle.
Up to 7 tentacle attacks when not amorphous
A half-farspawn half-illithid voidmind commoner 1 who doesn't use the amorphous ability from its templates possesses 2 tentacles, 4 tentacles, and 1 tentacle that attacks of its own accord because the creature already has other natural weapons, respectively. The DM must determine if both sets of tentacle attacks can be primary attacks or if the creature or the DM picks one set of tentacle attacks to be primary attacks. Because it's less complicated for an already complicated characters, this DM would allow all the tentacles to be designated as primary natural attacks.
With that in mind, the creature could take the full attack action to make up to 6 tentacle attacks at its normal attack bonus with its normal Str damage modifier. Or, were the creature wielding a longsword, its full attack could be 1 attack with the longsword at its normal attack bonus with its normal Str damage modifier then up to 6 secondary tentacle attacks, each at a −5 penalty and half its normal Str damage bonus. In either case,—before, after, or during, I guess—the creature can make the extra attack with its voidmind tentacle at its normal attack bonus with its normal Str damage modifier. (For more on natural attacks and their interactions with flurry of blows, manufactured weapons, and more, see this question.)
