You can not have more than (1) scheduled / open funding rounds at a time.
How many funding rounds you should have in total or how often should you raise money? This is up to you. We believe you know the best when is the right time for the next funding round to achieve the desired growth for your business. Just be careful, too many rounds might signal poor company planning and might be seen as unprofessional by the investors, and in addition, could annoy current shareholders.