For imaginary quadratic number fields F = $mathbb{Q}(sqrt{{varepsilon}p_1{ldots}p_{t-1}})$, where ${varepsilon}{in}${-1,-2} and distinct primes $p_i{equiv}1$ mod 4, we give condition of 8-ranks of class groups C(F) of F equal to 1 or 2 provided that 4-ranks of C(F) are at most equal to 2. Especially for F = $mathbb{Q}(sqrt{{varepsilon}p_1p_2)$, we compute densities of 8-ranks of C(F) equal to 1 or 2 in all such imaginary quadratic fields F. The results are stated in terms of congruence relation of $p_i$ modulo $2^n$, the quartic residue symbol $(frac{p_1}{p_2})4$ and binary quadratic forms such as $p_2^{h+(2_{p_1})/4}=x^2-2p_1y^2$, where $h+(2p_1)$ is the narrow class number of $mathbb{Q}(sqrt{2p_1})$. The results are also very useful for numerical computations.