Try: <b> =CONCATENATE( COUNTA(A1-A400), ” units”)</b>
Depending on what you need this for, you may also want to just keep the cell value as =COUNTA(A1-A400), but then format the cell so that it displays “units” after the number. To do this, right click on the cell, select “Format Cells…” and select a custom category. Enter in: <b>#” units”</b> (with the quotes) and hit OK. That way, the value of the cell will stay just a number — meaning you can easily use it as a value in other functions, for instance — but Excel will display ” units” after the number.
(Also, did you mean A1:A400 instead of A1-A400?)