3,2 million, WITHOUT touring Japan, Australia, New Zealand (countries included in Lovetown Tour, natural sequel of The Joshua Tree Tour).
Only 5 concerts failed to soldout: Las Vegas (aprox: 1,000 tickets unsold), Switzerland (aprox 10,000), Oakland (aprox 15,000) and Los Angeles (U2 sold 132,000 tickets but 10,000 remain unsold).
In Europe only 1-2-3 stadiums were booked in countries like France, Italy, Spain, Germany.. And only one concert in Scandinavia (in Sweden). U2 can esaily sold 500,000 more tickets in those countries.
Only UK an Ireland were well represent, with many stadium and arena dates.
U2 sold 1,1 million tickets in Europe in 1987 but could easily sold 2 million!
Vox