If your state of residence is NJ, all your income is taxable by NJ.
Money you earn by physically working in NY, or that you earn by working remotely for an NY-based employer for your own convenience (as opposed to the convenience of your employer) , is also taxable by NY. Your obligation to file a NY tax return is determined by the amount of your NY-source income, not by the number of days you spend working in NY.
https://www.tax.ny.gov/pit/file/nonresidents.htm
You'll be able to take a credit on your NJ return for taxes paid to NY, so you won't be double-taxed.
Note: A bill has been introduced in the New York Senate (Bill S-8386) which would designate remote work done outside the state due to COVID-19 as being for the convenience of the employer, and thus not taxable by NY - but the bill has not yet passed as of today (August 8).
**Answers are correct to the best of my ability but do not constitute tax or legal advice.
