Try to have 3 rigs at the workshop or in transit the whole time. That's the limit. You won't receive any e-mails while you have 3 rigs in the shop / in transit. When you finish a job and collect the money, you'll get another e-mail with another offer, and when you pick a job, the machine will always arrive the next day, so always try to leave the workshop with no rigs on it and 3 on transit for the quickest profit.