Limit of cash on hand

It depends in the turnover of the assessee

Cash in hand as per book means which books you talking about ?
cash in hand calculation depends upon different factors including his previous year income tax cash in hand also

Declaration of any amount of cash would not be a problem as long as it can be explained to be acquired by legal means and tax has been paid on the same in the current or any of the earlier years.

For example, if the assessee has filed an income tax return of around 3 lakh each year for the past 10 years, then, notwithstanding any other investments or acquisition of property, he can justify being in possession of (or having a book balance of) ₹15 lacs.

However, despite not being illegal, holding or declaring such large amount of cash is unadvisable because it increases your chances of scrutiny by income tax authorities that may lead to other avoidable complications.