First, go to the Questions tab and then click Frequent. The questions here are sorted by how many duplicates link to them. The top one is the most-linked-to. If the only reason you wanted to know the number of duplicates is to achieve this sorting, you're done.
If you actually want to know the number, view the question. Linked questions are listed at the side with a link at the bottom that says "see more linked questions." Click that and you'll be at a page like https://meta.stackexchange.com/questions/linked/86997?lq=1 which says, on the side, how many questions are linked to it.
This is slightly inaccurate in that it doesn't distinguish between linked as a duplicate target and just linked because someone mentioned it in a question, answer, or comment. However I believe it can serve your purpose.