Celebrating Lag Baomer. Lighting bonfires and going to Meron.

Going to Meron is definitely a custom (the whole celebration of Lag BaOmer is a custom), and according to [this article][1], over 500,000 people were expected to make the pilgrimage in 2009.

According to [this][2], as of Iyar 2011 there were 5,837,000 Jews living in Israel, so that's a pretty large percentage.

As for lighting bonfires, I don't have any numbers, but my understanding is that it is done all over Israel.

  [1]: http://matzav.com/over-500000-expected-in-meron-for-lag-baomer-5769
  [2]: http://www.jewishvirtuallibrary.org/jsource/Society_&_Culture/newpop.html