Look at the K-Map below :
Prime Implicant : the biggest subcube that should not be completely covered by any other subcube (some may be covered)
EPI - It should cover at least one element that is not a part of any other subcube.
Looking at the diagram, it is quite easy to say that $EPI = 2$ (red colored subcubes)
And $PI = 6$ $(2 EPI + AB + C'DA' + CDB' + A'B'D)$ -- two circles indicate a PI.
Watch this first. Then proceed by reading this and this_too.
Prabhanjan_1 Do you still think its $3$?
ABD' is covered by AB. So ABD' is not a prime implicant..