How many significant figures in 0 .00000000006?

Answer 1

One.

In scientific notation 0.00000000006 = 6 x 10-11 which has 1 digit in the mantissa (the bit before the x 10-11) showing the number has only 1 significant figure.

The first significant figure is the first non-zero digit; then count the number of digits from there onwards until just zeros remain and stop counting then. Trailing zeros are not normally counted as significant, but trailing zeros after a decimal point (which would not normally be written) could indicate rounding to significant figures and be counted, eg 3.5897 to 4 sig fig is 3.590