No one knows.
A useful definition of sentience due to the philosopher Thomas Nagel is 'something it is like' to be.
For example, we intuitively feel that there is nothing it is like to be a brick, but that there probably is to be a dog and so on.
However, there is no objective test currently known to physics which can tell if some other entity is having such 'first hand experience', and correspondingly no designs that will definitely lead to sentience.
The best test we have is the Turing test and its variants. The most obvious designs are neuromorphic ones, since we know that the design of the human brain is at least correlated with sentience.
In the light of the above, we can't definitively say a great deal about lower complexity thresholds for sentience - the best we can do count neurons in creatures that we might be prepared to admit are sentient.