Steam or smoke is seen pouring from the open manhole in Old Street in a shocking video taken at the scene. A pile of rubble is by the manhole cover as shocked witnesses gather at the scene. One person can be heard saying: “It exploded!” Another can be says: “What the f**k! Oh my God.” A third said: “That was scary. Did anyone get hurt?”
There were 4 witnesses
There were at least three witnesses, but there may have been more. 
( z0 person :quant 4 :ARG0-of ( z1 witness ) )
(z0 / person
    :quant 4
    :ARG0-of (z1 / witness-01))
There were 4 witnesses
z0 :quant 0
z0 :ARG0-of z1