This is a tip on how to to validate textbox control to enter minimum characters.
Select the column which you want to validate. Go to "Column validation" section of that column and enter Formula and Error Message as shown in the below figure. In the below figure we have added validation for "Title" column.
Once you have added the formula =LEN(Column name)>n where n for number of characters.
Then test the functionality by entering the characters, if it is not matching your requirement the error message will get display according to the "Error Message" field you have entered in the above figure.
Devendra Velegandla is a four-time recipient of Microsoft's Most Valuable Professional (MVP) award (2014-2017) for Office Servers and Services MVP and SharePoint Server. He received the MCC award for his contributions in Microsoft MSDN/TechNet forums. He loves sharing articles, videos, and tutorial on SharePoint and Office 365.