Data validation rules are triggered when a user adds or changes a cell value.
In this case, we need a formula that returns FALSE as long as entries in C6:C9 sum to a total equal to or below 1000. We use the SUM function to sum a fixed range and then simply compare the result to 1000 using less than or equal to. Note the range C6:C9 is entered as an absolute reference to prevent the reference from changing automatically for each cell that data validation is applied to.
Each time a number is entered, the validation is triggered. As long as the sum remains less than 1000, validation succeeds. If any entry causes the sum C6:C9 to exceed 1000, validation fails.
Note: Cell references in data validation formulas are relative to the upper left cell in the range selected when the validation rule is defined, in this case B5.