  In this paper, we prove that the number $B(p,g)$ of isomorphism classes of
abelian varieties over a prime field $\mathbb{F}_p$ of dimension $g$ has a
lower bound $p^{\frac{1}{2} g^2 (1+o(1))}$ as $g \rightarrow \infty$. This is
the first nontrivial result on the lower bound of $B(p,g)$. We also improve the
upper bound $2^{34g^2} p^{\frac{69}{4} g^2 (1+o(1))}$ of $B(p,g)$ given by
Lipnowski and Tsimerman (Duke Math. J. 167:3403-3453, 2018) to $p^{\frac{45}{4}
g^2(1+o(1))}$.
