The less exemptions you claim on your W-4, the more taxes that will be withheld from each paycheck. That means at tax filing time you will most likely not owe additional tax, and will most likely get some of the withheld taxes refunded to you. If you claim more exemptions, then less tax will be withheld from each paycheck. That will increase the possibility of owing additional tax at tax filing time.
I can't speak for state taxes. But for federal taxes it's important to read the instructions carefully. If you don't read the instructions then it's perfectly possible that you will claim more exemptions than you are entitled to, and will end up owing more tax at tax filing time.
If your employer did not provide you instructions with the new W-4, (they should have) you can get them yourself at https://www.irs.gov/pub/irs-pdf/fw4.pdf One common mistake I see that indicates the instructions were not read, is if an excemption is claimed in box E and the tax file does not have a qualifying dependent under the age of 17 on Dec 31 of the tax year. So please read the instructions for your own sake, and if your dependent will be turning 17 on or before Dec 31 of 2018, they do not qualify you for that specific exemption.
