the rule of thumb is that you want to have at least 150 to 200 conversion events per variant - so if you're trying to improve sign up rate, you'll want 300 to 400 total signups split between a two way test

if your conversion rate is 10% from visitors to signups (for example), then you'll need 3000 to 4000 total visitors

but the smaller the effect of your change, the more traffic (or power) you'll need to detect it