Dear Supriya,
Employee number are given based on DOJ,actually its a Primary Key(unique no) if you have got a HRIS(each employee is having unique EMPID),its automatically generated.In your case you can keep a record book where numbers can be entered incrementally,so that it wount create any problem.
Also Keep in mind no employee should be given a retired EMPID because it may create problem when some employees are going for a case or if company has any labour problem.
If you know MS Acess,MS SQL Server or Oracle design a database either opting for auto option so that next record no(EMPID ) will be created automatically
Choose either Numeric or Varchar as property of field ,if u are choosing numeric valu EMPID can only be numeric data like 1241 or 4120,if u choose varchar as datafield then you could name 142BGLR,142CHN etc,
Think about ur company future growth choose 4 dgit or 5 digit