Are there many mosquitoes in Costa Rica?
It depends on where you are exactly. Of course, being in the tropics you are bound to encounter a mosquito and many other interesting critters, but there are certain areas with more mosquitoes than others. For example, swampy areas like Tortuguero will have more, naturally, than the highlands in Monteverde, where temperatures drop low enough at night to make it inhospitable for them.
Either way, it’s a good idea to be prepared with some natural insect repellent and thin, long-sleeved clothes for dawn and dusk, which is when they are the most active.