How to convert string count into number?
Drupal doesn't provide any conversion of value in "Math Expression" field. So we need to fix it in another way, for example following these steps:


Add two new columns "string count" and  "field_number" same as previously added, excluding


When add fields tick "Exclude from display" checkbox
Select "Plain Text" to "Formatter" dropdown

After it, edit "Math-Expression" field, in "Expression" textarea write your mathematical expression with that newly added fields, e.g. [string_count_1] - [field_number_1]