As far as I know there are no limits in this area.  IRC 6695 has $25K limits on various other penalties but there is nothing mentioned regarding a limit on due diligence.  See paragraph (g):

https://www.law.cornell.edu/uscode/text/26/6695

There was someone who came through on this forum a few years ago who said she was assessed $30K+ in penalties and it put her out of business.  Later it came out that this was not her first DD audit AND she was reporting all sorts of documents on the 8867s that she "relied upon" but did not actually look at or have copies on file.