Yes, there is a limit to robots.txt file in terms of filesize 500 KB as Vipin points out.
A maximum file size may be enforced per crawler. Content which is after the maximum file size may be ignored. Google currently enforces a size limit of 500kb.
Source: Robots.txt Specifications
In terms of limitations (not sure if you meant that as well), there’s the fact that the robots.txt file is merely a optional directive, not a mandate. So search engines don’t have to adhere to your guidelines. It’s unethical not to, but it’s not illegal.
We’ve actually covered this queston in our FAQ here: Robots.txt file: the ultimate reference guide. You can read a lot more on the robots.txt file if you’re interested in learning more.
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