* Bilinguals, by definition, know two languages. This is their defining characteristic.
* Monolinguals know one language. This is also their defining characteristic.
So, bilinguals know more languages *by definition* than monolinguals.
However, the question might be trying to get at something else, like:
* Do bilinguals have an advantage in learning additional languages? There's evidence to suggest this might be true. Bilinguals often develop skills in language learning that monolinguals might not, such as an understanding of language structure and a more flexible cognitive style.
* Are bilinguals better at learning *any* new skill? This is an area of ongoing research. Some studies suggest bilinguals may have advantages in executive function (like multitasking and problem-solving), which could benefit them in learning new skills.
In short:
* Bilinguals know more languages than monolinguals because they know two, while monolinguals know one.
* Bilinguals might have an advantage in learning more languages due to their existing language skills and cognitive abilities.
It's important to remember that every individual is different, and these are general trends, not absolutes.