The t value for a 99% confidence interval estimation based upon a sample of size 10 is

For a sample size of 10, the t-value is about 3.25 (from tables) at a 99% confidence interval.

Explanation:
When the standard deviation for the population is not known, the confidence interval estimate for the population mean is performed with the Student's t-distribution.
The confidence interval for the mean is calculated as

where
  = sample mean,
s = sample standard deviation,
t = t-value (provided in tables),
n =  sample size.

When reading the t-value, (n-1) is called the df or degrees of freedom.