The competitive exclusion principle (CEP) is a fundamental concept in the niche theory, which posits that the number of available resources constrains the coexistence of species. While the CEP offers an intuitive explanation on coexistence, it has been challenged by counterexamples observed in nature. One prominent counterexample is the phytoplankton community, known as the paradox of the plankton. Diverse phytoplankton species coexist in the ocean even though they demand a limited number of resources. To shed light on this remarkable biodiversity in large ecosystems quantitatively, we consider \textit{intraspecific suppression} into the generalized MacArthur's consumer-resource model and study the relative diversity, the number ratio between coexisting consumers and resource kinds. By employing the cavity method and generating functional analysis, we demonstrate that, under intraspecific suppression, the number of consumer species can surpass the available resources. This phenomenon stems from the fact that intraspecific suppression prevents the emergence of dominant species, thereby fostering high biodiversity. Furthermore, our study highlights that the impact of this competition on biodiversity is contingent upon environmental conditions. Our work presents a comprehensive framework that encompasses the CEP and its counterexamples by introducing intraspecific suppression.