it will send a minimum of 20% of total daily sending volume of new leads. Your total per day is the sum of each of the senders max per day setting. you want 20 new contacts to start the sequence. We send every day at least 20% of that total sending capacity to new leads (i.e. they've never been sent to before).
Example: You have 1 email sender attached to a campaign that will send 100 emails per day. By default, we will send to 20 new leads (i.e. they will receive the first step in the sequence) and dedicated 80 emails to follow ups (steps 2 or greater)