Why are there two elements within the first period?

Hint: A component may be a pure substance consisting only of atoms that each of them have an equivalent number of protons in their atomic nuclei. Unlike chemical compounds, chemical elements can't be weakened into simpler substances by chemical means.

Complete answer:
There are two elements within the playing period because consistent with the physics, the 1s orbital of this era is filled. They follow a duet rule to determine which two electrons are present within the 1s orbital.
There are two elements only within the first period: hydrogen and helium. Period 1 elements follow the duet rule, they only need two electrons to finish their valence shell. These elements can only hold two electrons, both within the 1s orbital. Therefore, period 1 can have only two elements.
The first period has fewer elements than the other periods within the table. They are often explained why there's less elements within the first row in modern theories of atomic structure. This is often because in physics, this era fills up the 1s orbital.

Note:
The playing period of the table contains only 2 elements because the k shell gets completely filled at the end of the playing period and it can accommodate only 2 electrons. Period 1 elements follow the duet rule, they only need two electrons to finish their valence shell. These elements can only hold two electrons, both within the 1s orbital. Therefore, period 1 can have only two elements.