How do you get beta users for your new app and how many do you aim to get before initial launch

We are ready to go with our new app (iOS only at first) and the sign up page at Guidee.site, but we are unsure how to go about getting new beta users. I've tried targeting facebook groups around our target users to share but the moderators aren't accepting posts. Any suggestions on how to go about getting some beta users before we launch on app store? And how many do you usually aim to get? Thanks!