First, ponder on possible solutions. If the line fails to hit the sphere, there will be zero. If we pierce it, there will be two. If we just touch it, there will be only one.

Substitution? t^2 + 2^2 + 0^2 = 4

Then t^2 + 4 = 4

And t^2 = 0 ==> t = 0

Examine the point (0,2,0) -- Hmmmm. Only one?