How many "exemptions/allowances" should I put on W-4 if I don't want to owe taxes at end of the year?

Zero -- especially if you're not making the estimated tax payments that you should be on the Uber money.

file to withhold at SINGLE, ZERO EXEMPTIONS for maximum possible table withholding. You can even specify an additional dollar amount you want withheld for federal income tax for even lower net pay.

zero works

Probably 0 since you Uber part time and are filing separate, which costs you a lot of deductions and credits.

S-0. Your Uber income will be subject to self employment taxes. When it comes time to actually do your taxes, make sure you still have to file MFS. You may qualify to file HOH if you did not live with your spouse after 6/30/18. MFS has a lot of restrictions an is ineligible for some credits.

The fewer dependents you list on your W-4, the more they withhold each check as the tables assume you will owe more taxes. If you claim one less dependent on our W-4 then you will actually claim on your 1040 that increases the odds you will not owe any taxes. However, doing that is actually a violation of tax laws. Usually if the number varies by "1" the IRS says nothing about it. The difference usually does not trigger an alerts on your return.

If you want to LEGALLY assure that you will not own taxes, you can request that your employer withhold an additional amount above the tables says to withhold. If you owed money last year, take the amount and divide by 26 (if paid bi-weekly) and ask them to withhold that much additional from each check.

Usually the tables are pretty accurate and is you do not have any odd income during the year, what they withhold is usually the right amount. You may be within a couple hundred dollars of either getting small refund or owing a small amount.

I receive a small monthly pension from a former job. It is small enough that the tax tables say I would owe nothing on it. But when added to my regular income, it pushes me up a bracket and I owe about $800 every year. So I have my employer withhold an addition $40 from each check and it balances out when tax time comes around.

No one here has any clue how much tax you will owe for your self-employment (Uber). Will you be making quarterly estimated tax payments or are you trying to get THAT tax taken out of your W-2 job as well?

We are not clairvoyant.

Why does she do her taxes separately? You, as a couple, will pay more in taxes if you file separately than if you file jointly.

You can use www.paycheckcity.com to look at taxes (good free tax calculator).