With ND8 you are more likely to get something like a 1/64th second exposure in broad daylight. It is too weak for a 5 second exposure unless we are talking about extremes such as a combined very low ISO, very small aperture, and very overcast daylight.
It depends on the ambient(existing light), the desired aperture, and ISO that you would like to use of course. We can't just tell you what ND filter to use without knowing the rest. You likely would want something closer to 8 STOPS(ND256), and not ND 8 for a 5 second exposure in broad daylight.
For example, on an overcast day, this was shot at ISO 100, f/11, at 15 seconds, with a 10 stop/ND1024 filter: