Should I maintain my weight or is it ok to lose more?

No one but you can answer that, though the one person you could consider asking is your doctor.

You're at BMI 20.6, on a scale where 18.5 to 24.9 is normal weight. The normal BMI range doesn't fit everyone, but most people (maybe around 80%) will be at a healthy weight somewhere in that range. Some very muscular people might be at a good weight above 24.9. We don't know how muscular you are. Some very lightly built people (narrow pelvis, shoulders, small head, delicate limbs, etc.) might be OK below 18.5, though that's rare. We don't know your build.

Moreover, healthy weight for a given person is always a range, and we have personal choices for aesthetics, or related to our (sub)cultural context. We know nothing about your preferences or culture.

At age 18, you're at a difficult point. I can't speak for you, but at 18, other people's opinions had a lot of effect on me - probably way too much, honestly. At 18, you've probably just recently lost your gazelle-like pre-adolescent maybe-gawky but lovely-in-its-way body, and are growing into full womanhood - also a lovely thing - but maybe not yet 100% there.

That's a lot of personal change to absorb!

If you can - and I know this is asking a lot - it's a good time to think about how you want your life to be, and what body configuration/composition will best support that: Do you prioritize strength, culturally-driven attractiveness (which is a fickle thing!), athletic performance, health? What are your values? What woman do you want to be?

Ask your doctor what's healthy for your build. Ask yourself who you want to be. Pursue that, within the healthy range. If you find your own goals, your own targets, that's better. We can't tell you, your friends can't tell you: It's your path you want to optimize.