Depending on where in the world you are working, your local codes of practice should give guidelines.
Also consider what issues could affect concrete quality :
1) Change of worker shift as you mention
2) Different mixing plants or readymix suppliers if more than one
3) Difference in day/night working conditions and temperature
4) Cement from different silos/batches and aggregate stockpiles with different moisture contents etc
For every change that may give rise to a change in mix quality, you will neeed a batch of cubes. The batch should consist of probably 3 cubes for the statistical and contractual testing, plus at least 3 cubes to be tested at various early ages to pick up any major problems in time.
It may also depend on the cross-section of the structure, but I suggest at least one batch of cubes per 50 cubic metres of concrete.