Well, using SUBSTR alone can hardly ever give you any usable result with this problem ;).

TRANSLATE seems more appropriate. From your example I conclued you only have numerics, comma and decimal point and space characters in your strings. If so, you can use the following:

SELECT LENGTH(TRIM(TRANSLATE('12.5,44.7,400.25,8.5', ',.0123456789', ','))) FROM dual;

It will return you the number of commas in your string.