Nikon L21 has a 8.0MP 1/2.3-inch (6.08 x 4.56 mm ) sized CCD sensor and features Expeed C2 processor. On the other hand, Panasonic ZS8 has a 14.0MP 1/2.3-inch (6.08 x 4.56 mm ) sized CCD sensor and features Venus Engine FHD processor.
Panasonic ZS8's sensor provides 6MP more than Nikon L21's sensor, which gives a significant advantage in real life. You can print your images larger or crop more freely.
On the other hand, please keep in mind that Max sensor resolution is not the only determinant of resolving power. Factors such as the optical elements, low pass filter, pixel size and sensor technology also affects the final resolution of the captured image.
Below you can see the L21 and ZS8 sensor size comparison.
Nikon L21 and Panasonic ZS8 have sensor sizes so they will provide same level of control over the depth of field when used with same focal length and aperture. On the other hand, since Nikon L21 has 40% larger pixel area compared to Panasonic ZS8, it has potential to collect more light on pixel level hence have less noise in low light / High ISO images.