It's hard to escape toxins in our environment. With radon seeping up from the ground, chemicals in our cleaning products, synthetic carpets releasing VOCs, or even a diesel truck idling outside your window - we are constantly exposed to some level of toxins in our environment. But thankfully, plants can turn many of those toxins into clean fresh air for our home.
In 1989, NASA did a study on the air-purification abilities of plants (to clean the air in space stations), and found that plants could naturally remove harmful environmental toxins such as benzene, formaldehyde and trichloroethylene from the air, helping neutralize the effects of 'sick building syndrome' - a syndrome where toxins build up in our modern air-tight homes.
There was not a specific number of plants suggested, but if you don't have any plants, buy a few now and learn which plants you enjoy keeping. NASA generated a list of plants and what they purify, which you can use as a guide, but I think any houseplants are an improvement, and it's always best to go with something that is easy to get you started. See the NASA list of air-filtering plants.
If your company allows it, having a small plant or two at your station at work can be energizing and reduce fatigue.
Interestingly, plants are also good at relieving stress, and improve your mood, so there are quality of life benefits beyond the clean air.