Shared virtual environments are becoming essential platforms for collaborative interaction and immersive entertainment, enabling users to be co-located and engage in activities together. The presence of surrounding virtual humans forms an environmental crowd, serving as a component of ambient stimuli in these environments. However, it remains unclear how crowd size affects users under different cognitive and motor demands. This study investigates the influence of crowd size on user performance, experience and social presence across three fundamental VR tasks: Spatial Locomotion, Memory Search, and Motor Coordination. We conducted a controlled within-subjects experiment, manipulating each task's crowd size at Small, Medium, and Large levels. Our results show that crowd size significantly impacts user performance, experience, and social presence, but these effects are task-dependent. While Medium size can enhance performance, Large size in cognitively demanding tasks may induce attentional blindness and diminish sensitivity to social cues. Task functionality further shapes how users perceive and respond to virtual crowds. Additionally, users' preferences for crowd size varied across different tasks, and most participants expressed a desire for control over the number of visible avatars. These findings provide novel insights into human crowd perception mechanisms, revealing cross-task perceptual variations that pave the way for further exploring crowd perception in shared virtual environments.