Don't send spam.
Don't send messages too fast: 12 per hour (1 message every 5 minutes) is more often than not fast enough.
Try to make the content of your message feel personal. For example, use the first name of the recipient if possible.
Don't send messages outside the recipient's regular business hours.
What is spam?
Sending messages in bulk doesn't necessarily mean you are sending spam. If the recipients of your campaign expect to hear from you, then by definition you are not sending spam even if you are contacting these people in the thousands using automated means.
However, if you are using WhatsApp to promote a product or service to people who have never heard from you and a message to their personal numbers is the first time they see you, then you are spamming people. Don't violate WhatsApp's Terms of Service by doing this. You will be putting your account in danger, too.
If enough recipients of the campaign you sent mark you as spam on the WhatsApp application, then your chances of getting your number banned increase significantly.