There is no hard and fast rule on this. One open house is not bad...I think more than 3-4 is pretty excessive but depends on the area, the price range, etc. Just my opinion.
As a new realtor, you’ll find that different agents have different approaches to open houses. Some host one every week, and others might do multiple a day, depending on their schedule and the market. I like to host at least one open house a month on the weekend to give the property good exposure without overwhelming myself. It’s important to find a balance that works for you and your clients while also being mindful of the time and effort it takes to run a successful open house.