Setting a frequency cap for online advertising campaigns is almost always recommended in most situations, unless you're running remarketing or brand awareness campaigns (where CTR is not your KPI) (AdGlare)

3/24 is the generally the best frequency cap for some unfathomable sociological reason, as on average more users will click on your ad if they see it 3 times per day. (The Online Advertising Guide)

If the videos you set a cap for are in other video campaigns, impressions from those campaigns for a given user will count towards the frequency cap. This only applies to in-stream and bumper ads. (Instapage)