Hello Simon,
the number of trades depends really on the EA itself. Usually, the more entry conditions you have, the fewer trades you will see.
I am not sure if I get your next questions, but when I track the performance of the EAs, I look at the last 2 weeks and the last 1 month. So I want my EAs to be still profitable in the shorter and in the longer term.
I would not use more than 300k bars for M1 and more than 150k on M15, and more than 50k on H1.
Kind regards,