Is there are max number of creatives I can test per campaign?

Technically, no there is no limit to how many creatives you can test per campaign. However EH would make two recommendations:

1. If you want campaign level metrics, include a minimum of 4+ creatives in the test. Essentially the report will aggregate the results from the 4+ tests into ‘overall’ campaign scores. If you include less than 4 you’ll just see the individual creative scores in the report.
2. There is no max, however things can start to look a little busy on the page if your report is reporting results for more than around 12 creatives. In those circumstances, EH is happy to tweak the reporting style to accommodate the higher volume.