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The Count expression counts the number of non-null rows. If that and the "Number of Rows" field give different amounts, it is because (as you found out) you have a number of nulls in your data - 866 in your case.
When using data from a CSV, Tableau uses a number of rows to determine the field type. Once determined, it uses that type for that field. If it has determined "Amount" is a number, then if there are any strings, it will attempt to turn them into numbers. However, if it cannot do so, you get a NULL record instead.
If you change the field type inside Tableau to be a string, you will still have NULL, as this is a further type change on top of the original, so you will need to use other fields to determine which rows to look at in your dataset.
Have a look through the CSV. I am guessing that there is something in the amount column that is not a true number - perhaps a currency sign?
Hope this helps,