No, you can use as many ad networks as you want. For example, if you want to maximize your revenue, you may want to use Google Ad Manager as a primary one, and using the “Ad Failed To Load” block you can then invoke other ad networks like StartApp to fill that slot.
That notice means that Google Ad Manager is intended to be used with high ad volume. If you have, let’s say, less than 1,000 impressions per day, you should consider other ad networks.