The number of elements in a power set of size <= 1 is the size of the original set + 1 more element: the empty set .

Suppose $|A|=m$ . What is the size of this set:

$$| \{X \in \mathscr{P}(A) \in : |X| \leq 1 \} |$$

Let's start with a specific example:

$$| \{X \in \mathscr{P}(\{1,2,3\}) \in : |X| \leq 1 \} |$$

First, let's create the power set, stopping once we hit an element whose size is greater than 1:

$${\mathscr{P}(\{1,2,3\})} = \{\emptyset, \{1\}, \{2\}, \{3\} \}$$

Notice that the number of elements of size $\leq 1$ is the size of the original set + 1 more element: the empty set.

Thus:

$$| \{X \in \mathscr{P}(A) \in : |X| \leq 1 \} | = m+1$$