Fix type argument count check for Instances

Currently for a class defined as
```python
T = TypeVar("T", bound=int)
S = TypeVar("S", bound=str)
Ts = TypeVarTuple("Ts")

class G(Generic[T, Unpack[Ts], S]): ...
```
we allow a least 2 type arguments, but in fact 1 type argument may be fine as well, for example if it is an unpack of fixed (or maybe even variable) length tuple, or an unpack of TypeVarTuple (bound in current scope).