You only gain bonus talents once
Magic Talents says:
As a caster gains levels, they gain magic talents. Magic talents, like feats, may be spent to allow a caster to gain new powers and abilities.
The number of magic talents a caster gain differs between classes, but all characters gain two bonus magic talents the first time they gain a level in a casting class, regardless of which class is chosen. (...)
So every time your caster level increases, you may gain a new talent (depending on the class). For some classes, that means one talent per class level, for others it is a bit slower. For instance, the Thaumaturge says:
The Casting class feature enforces this and say that you only gain those bonus talents once, when you first obtain this class feature.
A thaumaturge may combine spheres and talents to create magical effects. A thaumaturge is considered a High-Caster. He may use either Intelligence, Wisdom, or Charisma as his casting ability modifier and must make this choice at 1st level. Once made, this choice cannot be changed. (Note: All casters gain 2 bonus talents and may select a casting tradition the first time they gain the casting class feature.)
This means that the first time you pick a class that has the Casting class feature, you gain two bonus talents because of the Casting class feature, and whatever additional magic talents the class gives you. These magic talents (and not the bonus talents) will be granted when multiclassing into this class.
If you take a look at the Hedgewitch table, you will notice that she only gains 2 talents, because she starts with a caster level 0. But the number is listed in parenthesis, because it only affects those characters who are taking the Casting class feature for the first time, for everybody else (multiclassers) they gain 0 magic talents at level 1.
On the other hand, the Incanter is the opposite, they gain two bonus talents for obtaining the Casting class feature for the first time, and two talents for taking the first level on the class, for a total of 4 talents if this is your first class, and two talents if you are multiclassing into it.