An answer on a Stack Exchange site should be self contained, complete and (duh) answer the question. That excludes that you are free to offer your services in all your answers.
In the Help Center the article on promotion says:
Don't talk about your product / website / book / job too much.
you must disclose your affiliation in your answers.
So if you answer a question where you can naturally include a reference to your consultancy work you're probably fine, if you end every answer and question with "Hire me!" you risk the post being flagged as spam and that will have severe consequences up to the point that your account might be deleted.
You might wonder if comments are a good fit to do this promotion but comments are only meant to either ask for clarification or point out issues with a post. They are not for promotion either.
if you want to make people aware of your services, use your about me. It is the place where you can tell about your self and promote stuff (within reason, as always).