As a general rule of thumb, a good headlamp should have at least 50 lumens. This is enough light to see clearly in most situations. However, there are some specific circumstances where you may need more or less light. For example, if you're trying to read a map at night, you'll need more light than if you're just trying to find your way around in the dark.
There are a few different factors to consider when choosing how many lumens to get in a headlamp. The first is the distance that you want the light to reach. The further the distance, the more lumens you'll need. The second factor is the amount of time that you'll be using the headlamp. If you only need it for short periods of time, then a lower lumen count will be fine. However, if you're going to be using it for long periods of time, such as overnight camping trips, then you'll want a higher lumen count so that your eyes don't get fatigued from looking at a bright light for too long.
The last factor to consider is the type of batteries that the headlamp uses. Some batteries can power headlamps with high lumen counts for much longer than others. So, if you know that you'll be using your headlamp frequently and for long periods of time, it's worth investing in a headlamp that uses high-quality batteries.