In the United States to be considered a savings account the number of monthly transaction is supposed to be 6 or fewer. This is covered in Federal Regulation D.
allow no more than six transfers or withdrawals per calendar month or
statement cycle of at least four weeks for the purpose of transferring
funds to another of the depositor’s accounts at the ...
I know it's a very old question, but there are some new services which are providing single use cards which will be very helpful in such cases.
They are single use only and you can also associate a transcation limit to them.
Some of the examples are: