My offer letter states that the tax shall be deducted at the source. How much will they cut from my monthly salary?

Based on an IT declaration that you have to file every year about your tax saving instruments (80C, 80G etc.). Accordingly, they will calculate your tax liability and deduct on a monthly basis.

That is applicable for employees since the employer is obliged to deduct tax at source on the basis of estimated income. For retainers, employed as consultants, tax would be deducted at 10% flat rate under section 194J

So if a person is earning 1lakh per month, in hand would be 90k?

yup

Yes, that is correct. Its a 10% TDS.

Bump