Each purchase (including dividend reinvestments) is treated as its own separate event for purposes of Form 8621. Calculations for this form are on a per share basis.
Please note that this form is not included in any version of TurboTax. However, you can complete the form separately and attach it to a return prepared in TurboTax to be printed and mailed.
It is recommended to use the download (or CD) version of the program when using unsupported forms to manually make any necessary adjustments to Form 1040.