Does an LLC qualify for the 20% deduction? Is there a maximum income limit for a professional to get this deduction (dentist)? If there is a max. income limit, is the income gross income of net income after expenses and deductions? Can I use Turbotax online self employed version or another version to file my taxes after I set up LLC?
You don't need anything special like a LLC. Any business (besides a C-corporation) potentially qualifies for it.
You will qualify for it if (1) the "taxable income" on your tax return (Line 43 of Form 1040) and (2) your business profit is under $315,000 (Married Filing Jointly) or $157,500 (Single). After that, the deduction starts getting phased out over the next $100,000/$50,000.
Although most businesses can still keep the 20% reduction if the income is above those amounts and they have enough W-2 wages or 'qualified property', several categories do not qualify for that provision. Unfortunately for you, the "health" field is one of those unqualified businesses. But again, if your income is below those dollar amounts, you will qualify for that deduction.