Boudary values concrete example

Let's suppose we have the following equivalence partitions:

|invalid partition A| valid partion B | invalid partition C
...................1|2              10|11................ 

As we see valid partion B is [2..10] (that is including 2 and 10). To test B partion let's take 7. Ok, now let's test boundary values. As we see boundaries are 2 and 10. In some books they say that besides 2 and 10 it is necessary to take additional points +- sigma.

So the question, how many values must we take to test valid partition B three (2, 7, 10) or five (2, 2.01, 7 , 9.99, 10)?