As of a few weeks ago, advertisements for JPMorgan Chase were appearing on about 400,000 websites a month. It is the sort of eye-popping number that has become the norm these days for big companies that use automated tools to reach consumers online. Now, as more and more brands find their ads popping up next to toxic content like fake news sites or offensive YouTube videos, JPMorgan has limited its display ads to about 5,000 websites it has preapproved, said Kristin Lemkau, the bank’s chief marketing officer. Surprisingly, the company is seeing little change in the cost of impressions or the visibility of its ads on the internet, she said. An impression is generally counted each time an ad is shown. The change illustrates the new skepticism with which major marketers are approaching online ad platforms and the automated technology placing their brands on millions of websites. In recent years, advertisers have increasingly shunned buying ads on individual sites in favor of cheaply targeting groups of people across the web based on their browsing habits, a process known as programmatic advertising — enabling, say, a Gerber ad to show up on a local mother’s blog, or a purse in an online shopping cart to follow a person around the internet for weeks. But as the risks around the far reaches of the web have been cast into stark relief, some advertisers are questioning the value of showing up on hundreds of thousands of unknown sites, and wondering whether millions of appearances actually translate into more sales.