Hi Vichian Pattarakeeratiworrakul,
You can achieve this by using a helper entity and a custom button. When users select items in the DataGrid and click your custom “Acknowledge” or “Proceed” button, the microflow behind that button can take the list of selected objects as input. Inside the microflow, you can use the "list length" function to get the total number of selected items (for example,"length($selectedList)" ). Then, store this count in an attribute of your helper entity and refresh the data view that shows it. This way, when users select items and click the button, they immediately see how many records they are about to process.