If you have so many plaques that they overlap, you need to dilute the phage stock even more, another 10 or 100 fold.   Otherwise, take your best guess or do not let the plates grow so long and count the plaques when they are smaller.  

As Gregory pointed out, dilute the sample to have decent but not too high a number. In a 10 cm diameter plate, a good range to get reliable information 50-200 plaques.Going too low or too high compromises accuracy.

If you see a lot of overlapping on your plaque, your sample is to concentrated to obtain good counts. If overlapping is isolated, I usually consider overlapping PFUs as one count. 

Hope this helps! 

Thank you very much...