It's not all about the quantity of emails you gather, you can scrape thousands from the internet, but these people will be very untargeted, and most people these days are almost immune to spammy sales emails.
If you're serious about bilding a list, build some kind of free resource in whatever niche it is you're interested in, provide VALUE, then offer something mindblowingly awesome for free, in exchange for an optin. This way you have a list of targeted followers who trust you.