Mfcttrf

Another awk solution:

awk -F" '
 OFS=""";
 for ( i = 1; i <= NF; i++ ) 
 if ( i % 2 == 0 ) 
 gsub(/,/, "", $i)
 
 
1' input.csv

This will use the double quote as a field separator and loop through all fields. If the field number is an even number (which is not fool-proof, but given your example it should mean that the field exists between quotes) it will remove any commas from that field. The 1 will cause awk to print everything (with the changes made) using the double quote as the output field separator.

In use:

$ cat input.csv
,7/30/2019,7/31/2019,Wed,8/1/2019,FH/FN 30yr & 20yr TBA & Spec ,"10,000",8/13/2019,
,7/30/2019,7/31/2019,"100",FH/FN 30yr & 20yr TBA & Spec ,"10,000,000",8/13/2019,
,7/30/2019,7/31/2019,"Jack, Mary, and Jane",8/1/2019,"123,456,789,012,345,678","10,000",8/13/2019,
$ awk -F" '
> OFS=""";
> for ( i = 1; i <= NF; i++ ) 
> if ( i % 2 == 0 ) 
> gsub(/,/, "", $i)
> 
> 
> 1' input.csv
,7/30/2019,7/31/2019,Wed,8/1/2019,FH/FN 30yr & 20yr TBA & Spec ,"10000",8/13/2019,
,7/30/2019,7/31/2019,"100",FH/FN 30yr & 20yr TBA & Spec ,"10000000",8/13/2019,
,7/30/2019,7/31/2019,"Jack Mary and Jane",8/1/2019,"123456789012345678","10000",8/13/2019,

NOTE: This will remove the commas in fields that are not numbers. In order to read this file correctly as a csv you will need to do that. If for some reason you want to retain those commas you can use the below solution.

awk -F" '
 OFS=""";
 for ( i = 1; i <= NF; i++ ) 
 if ( i % 2 == 0 && $i ~ /[0-9]/ ) 
 gsub(/,/, "", $i)
 
 
1' input.csv

Try for example awk:

cat oldfile | awk ' print gensub ("(,"[0-9]+),([0-9][0-9][0-9]),?([0-9][0-9][0-9])?,?([0-9][0-9][0-9]),?","\1\2\3\4","g");' > newfile

This works for large numbers, too.

Explanation:

awk is a programmable filter. The command given here in the commandline (between the outer single quotes "'") will be executed for every line of input from your file.

The awk program looks like this (different formatting):

 print gensub ("(,"[0-9]+),([0-9][0-9][0-9]),?([0-9][0-9][0-9])?,?([0-9][0-9][0-9]),?",
 "\1\2\3\4",
 "g");

The awk-builtin command gensub replaces things given in the first argument, with the replacement given in the second. If the third argument is a string starting with "g" or "G" it will replace all occurrences (tries until no more are found).

What is replaced? The first argument is a regular expression (q.v.) in double quotes, here are the parts: , then afterwards [0-9]+ which means a digit 0-9 repeated one or more times (postfix operator +) then , which is just a character, then [0-9][0-9][0-9] and a comma , followed by a question mark ? (you know what the first part means now but the postfix ? is new - the comma digits can be omitted). Then more digit groups and commas which may be omitted - this is for larger numbers.

In this explanation I have left out the parentheses ( and ) so far! These mark those things that are matched by the expression but remembered. In the second argument to gensub we reference the first 1 through fourth 4 things that were matched (the numbers) and print them out again here.

Assuming this is properly formatted CSV (the example data looks ok in this respect), we can use csvformat from csvkit to temporarily change the field delimiters to some other character not otherwise present in the data, such as @, delete all commas, and change the field delimiter back to the default again:

$ csvformat -D '@' file.csv | tr -d , | csvformat -d '@'
,7/30/2019,7/31/2019,Wed,8/1/2019,FH/FN 30yr & 20yr TBA & Spec ,10000,8/13/2019,

The output does not have quotes around the field that we modified, but that's because it no longer needs it.

Obviously, "deleting all commas" may delete commas that we don't actually want to delete, so we can be a bit more selective and only delete the commas in the 7th field:

$ csvformat -D '@' file.csv | awk -F '@' 'BEGIN OFS=FS gsub(",", "", $7); print ' | csvformat -d '@'
,7/30/2019,7/31/2019,Wed,8/1/2019,FH/FN 30yr & 20yr TBA & Spec ,10000,8/13/2019,

Try this:

sed 's/(".*),(.*")/12/' file

Your own attempt sed '/"/,/"/s/,//' fails because the address range you give only filters for a range of lines, not a range inside a line.

This type of task is nasty in standard sed. If it's just about one comma, then sed -E 's/("[0-9]*),([0-9]*")/1 2/ would do the trick, but for multiple comma you'd have to loop, giving ugly results like

sed -Ee :loop -e 's/("[0-9 ]*),([^"]*")/1 2/;tloop'

The ("[0-9]*) matches the opening double quote followed by any number of digits and will be referred to as 1 in the replacement, the ([^"]*") matches anything after the comma until the closing ", so 1 2 is the same, but with the first comma replaced.

Now the t command branches to the loop mark if a replacement was made. This gets repeated until there is no comma left to be replaced.

This even works for cases with more than one number with as many commas as you like: ,7/30/2019,"99,999,999,999,999",0,1 ,"10,000","foo, bar" will get transformed to ,7/30/2019,"99 999 999 999 999" 0 1 "10 000" "foo, bar"