[tex]55 \; \text{e}^{-}[/tex], [tex]55 \; \text{p}^{+}[/tex], and [tex]74 \; \text{n}^{0}[/tex]- in case cesium-129 is what you were referring to.
Cesium has atomic number [tex]55[/tex] as seen on a modern periodic table. A cesium atom nucleus would thus contain an equal number of protons; in addition to that, a neutral atom of cesium would contain the same number of electrons in its electron cloud.
For a particular isotope,
[tex]\text{Mass number} = \text{Number of protons per nuclei} + \text{Number of Neutrons per nuclei}[/tex].
This isotope of cesium has a mass number of [tex]129[/tex] and contains [tex]55[/tex] protons in each atomic nucleus. One cesium nuclei would therefore contain [tex]129 - 55 = 74[/tex] neutrons.
