GMB post frequency plays a role in your listing's visibility, however it seems to have diminishing returns past a certain point. You should be posting to your GMB listing at least twice a week and consistently, but any more than that is probably not going to make a difference to your ranking. Try to spread out the types of GMB posts you make between Offer, Event and Standard GMB posts. Create events for actual events that your business has or takes part in and make Offer posts when you have a sale or deal on your product or service.