Back in 2016, Baidu
Impacted by the Wei Zexi incident, Baidu limited the number of ads up to 4, and changed the label of ads from ‘promotion’ to ‘ads’.
3 years past, let’s see how Baidu ads are looking in 2019, and how many ads position in SERP now.
Case A: Ads block with a background.

This kind of ads block is easy to identify, which have a different background color. Ads will be shown on both the top and the bottom of the SERPs, up to 4 different ads.
Case B: Ads block without background.

This kind of ads looks more close to the organic search result. They can only be identified by the label ‘广告’ (Ads). The ads will be only shown on the top of the first SERP. But there will be up to 5 ads.
And in the second case, the first screen will be full of ads like below.

Users have to scroll down the page to see the organic result, which is not a good experience.