It is impossible to say definitively which has a bigger population: crabgrass, mice, or foxes.
Crabgrass is a plant, and plant populations are difficult to measure because they spread through seeds and can form massive patches. There are many species of mice and foxes, and their populations vary by location, environmental factors, and time of year.
To compare populations effectively, you need to specify the species, geographic area, and time frame. Population density—the number of individuals per unit area—is often more useful than total population numbers.