Not sure why you're being downvoted. BofJ does not hold 90% of Japanese stocks, just 90% of stocks in the Nikkei 225 index. Still interesting but very misleading title.
it's not even that they hold 90% of stocks in the Nikkei 225.
It's that among 90% of the stocks in the Nikkei 225 (about 203 stocks), the BoJ is one of the top-10 stockholders. Typically with large US stocks, the top 10 stockholders hold about 1-6% [0] -- so that implies, if the countries are substantially similar, that the BofJ holds 1-6% of 90% of Nikkei 225 stocks, or about 0.9-5% of total stocks in the Nikkei 225.
[0] http://finance.yahoo.com/q/mh?s=CSCO&ql=1 -- type in any large stock you can think of; you'll probably see Vanguard, T.Rowe Price, State Street, and a couple of other investment firms in the 1-6% range.
>the monetary authority’s exchange-traded fund purchases have made it a top 10 shareholder in about 90 percent of the Nikkei 225 Stock Average