how i can generate a unique number, which should start from 01 every day and should get incremented once a new item will get created in the list.
ex: today 2 list items are created so the number counter should be set to 02. tomorrow it should set back to 01 and so on.
This serial number should be available at the time of new item creation.
nintex forms office 365
Have you considered using the column named "ID"?
The Id column is already providing this feature on your SharePoint List.
The problem is that this number is generated once your save the item.
The problem of your desired feature is concurrency. Think in the case that to user are going to create a new item.
Both of them will see the same serial number.
You could have a counter that you increment (obviously) and every time the workflow runs check the current date against the last record. If the date has changed then reset the counter.
What happens when the first item created on a specific day gets deleted before the 2nd item is created, should the counter restart at 01 or continue from 02.
Generating counters like this is basic design flaws that you should get away from at-least in SharePoint terms.