According to the Central Limit Theorem the sum or average of at least 30 independent random variables with the same distribution is approximately normal. Therefore the answer is 'at least 30 measurements.'

Actually Welcome to the concept of expo functions.
f(x) = -8(2)^x - 12 ,
for f(0) ,here substitute x = 0
so we get as ,
==> f(0) = -8(2)^0 -12
==> f(0) = -8-12
==> f(0) = -20
hence, f(0) = -20