The term 'more or less mound-shaped and somewhat symmetric' indicates that the sample distribution is already somewhat towards normal distribution. According to Central Limit Theorem (CLT), if the sample size is large enough and have the distribution in the mound-shaped size, then the sample distribution will be approximately distributed.
There is no actual specific value to determine how large enough a sample should be. However, many literatures and experts suggests that the value should be at least 30 or more
