Campaign with no conversions
For campaigns with no conversions at all, it is better to test Maximise conversions with a Target CPA – Cost per acquisition.
In these campaigns, in order to drive more conversions, add new and more keywords and text different match types to extend your reach and demand.
Campaign with only a few conversions
If you have several campaigns with less than 30 conversions per month, consider merging these into one campaign, and apply the Maximise conversions with a Target CPA – Cost per acquisition strategy to that campaign (no need to choose a portfolio bidding strategy).
Alternatively, set up a portfolio strategy with your Target CPA and apply it to a number of campaigns that together have around 30 conversions per month.
Campaigns limited by budget
For campaigns with more than 30 conversions per month but limited by budget, it is better to test the Maximise conversions bidding strategy with no Target CPA.
Alternately, to get the extra budget for testing, relocated shared budgets from other campaigns, or pause campaigns that do not perform.
Extract from my Book ‘Making Google Ads Work’.