Counting the occurrence of a specific letter within a string (QGIS)?

array_length(
    array_filter(
        string_to_array("FIELD_NAME"),
        @element='Green'
    )
)

Change FIELD_NAME into your field name. If string is separated by another character, you should add that character to string_to_array function as seperator. For example:

string_to_array("FIELD_NAME", ';')

For Urban,Green,Urban,Green,Green string:

• string_to_array("FIELD_NAME") returns ['Urban', 'Green', 'Urban', 'Green', 'Green']
• array_filter(ARRAY, @element='Green') returns ['Green', 'Green', 'Green']
• array_length returns 3

Here's a way to find the number of Gs. Replace everything that isn't a G with an empty string, and count the length of what is left:

length(regexp_replace('Urban,Green,Urban,Green,Green','[^G]',''))
-> 3

length(regexp_replace('Urban','[^G]',''))
-> 0

[^G] is a regexp pattern that matches any character that isn't a G.

This expression should work:

array_length( 
    array_remove_all(
        array_foreach( 
        string_to_array( "column_name" ),

            array_find( string_to_array( @element ), 'Urban' )

        ),
        -1
    )
)

It turns turns a comma separeted sequence of strings, for example Urban,Green,Urban,Green,Green into an array.

Then it checks each element if it contains the string 'Urban', outputing an array where -1 means the element is not the string 'Urban'.

Then it creates a new array without the -1 elements, and finally counts the length of that array to get the instances of the search string.

Just replace the string 'Urban' with whatever you want to count.