How many pictographic letters are estimated to be in the Harappan script?

Why is the Indus Valley Civilisation sometimes called the Harappan Civilisation?

What evidence is there to establish that the people of Mohenjodaro had a great concern for Sanitation and Town planning?

Read the following excerpt and answer the questions that follow:

"Although the script remains undeciphered, there is widespread consensus on the direction of writing in the script. Barring a few exceptions, the writing direction is predominantly right to left (i.e., left to right in seals and right to left in the impressions)."
(Source: https://www.harappa.com/sites/defau/t/files/pdf/The-/ndus-Script.pdf)

(a) What is the primary medium where Harappan script is found?

(b) Why is the Harappan script significant in understanding their culture?