Assuming you are talking about common mollies and common tetras (both freshwater fish), then a general rule of thumb is that you can keep 1 inch of fish per 1 gallon of water. This means that for a 10 gallon tank, you could keep up to 10 mollies or 10 tetras. However, it's important to remember that this is just a general rule and there are other factors to consider when stocking your tank, such as the type of fish, their activity level, filtration system, etc. Therefore, it's always best to do some research on the specific fish you want to keep before adding them to your tank.