According to Kikar
, Rav Yitzhak Yosef Shlit"a said that to say this special beracha of:
ברוך אתה ה' אלקינו מלך העולם חכם הרזים
1) You must be 100% certain that there are 600,000 people present and that the news reports are accurate.
2) To say it with Shem Umalchut, you must be from a high area where you can see 600,000 at once.
Rav Yosef said that if you cannot see 600,000 people at once, you should say it without Shem Umalchut (i.e., just say: ברוך חכם הרזים).
According to another Kikar article
, Rav Yitzhak Zilberstein Shlit"a said that to get out of the doubt of whether there truly are 600,000 and whether you need to see them all at once, he suggests that you say the Beracha by reading it from the Halacha, e.g. from the Rambam