The requirement given in the image specifies a field asking for the number of group members, which can be anywhere from 2 to 20 visitors. To test this requirement, we can use boundary value analysis, which is a specification-based test technique that involves testing the values at or near the boundaries of an equivalence partition. An equivalence partition is a set of values that are expected to be treated in the same way by the system under test. For example, based on the requirement, we can identify two equivalence partitions for the input group size:

EP1: Group size < 2 (invalid)

EP2: Group size = 2 (valid)

EP3: 2 < Group size < 20 (valid)

EP4: Group size = 20 (valid)

EP5: Group size > 20 (invalid)

The boundary values for each equivalence partition are the values at or near the edges of the partition. For example, the boundary values for EP1 are -1 and 1. The boundary values for EP2 are 1 and 2. The boundary values for EP3 are 2 and 19. And so on.

To test this requirement using boundary value analysis, we need to select one value from each boundary and test it with different combinations of valid and invalid inputs. For example, we can select the following values:

BV1: Group size = -1 (from EP1)

BV2: Group size = 1 (from EP1 and EP2)

BV3: Group size = 2 (from EP2 and EP3)

BV4: Group size = 19 (from EP3 and EP4)

BV5: Group size = 20 (from EP4 and EP5)

BV6: Group size = 21 (from EP5)

We can then create test cases using these values and different combinations of valid and invalid inputs. For example:

TC1: Group size = -1 -> Invalid input

TC2: Group size = 1 -> Invalid input

TC3: Group size = 2 -> Valid input

TC4: Group size = 19 -> Valid input

TC5: Group size = 20 -> Valid input

TC6: Group size = 21 -> Invalid input

Therefore, we need a minimum of 6 valid test cases to achieve 100% boundary value coverage based on input group size.

You can find more information about boundary value analysis in [A Study Guide to the ISTQB® Foundation Level 2018 Syllabus], Chapter 4, Section 4.2.