Do I need fiber internet or is ADSL enough for video conferencing?

ADSL works, yeah, but it really depends on whether you're sharing the connection with other people at home or if you've got other stuff running - upload is usually the bottleneck. If you've got stable speeds, you can go with it, but fiber gives you way more headroom so you don't get that annoying lag in the middle of a meeting.

Upload is really the culprit here, and a lot of people don't realize it. Typical ADSL offers decent download speeds, but upload usually tops out around 1 Mbps or less. 1080p video conferencing with reasonable quality needs something like 2.5 to 4 Mbps of upload, so you're already done for. If you're at lower resolution (720p or less), maybe you can make it work, but then you get that pixelated image, freezing up whenever there's a lot of movement. Fiber normally offers symmetric upload or at least way more balanced speeds, so it makes a massive difference.

The second important thing is to actually test your current connection. Run a speedtest and see what upload speed you're actually getting guaranteed. Then try doing a video call during peak hours (late afternoon, evening) when the network tends to get congested. Sometimes the speed you're paying for is one thing, but what you can actually use is something completely different, especially with ADSL in crowded urban areas.

If you only do video conferencing occasionally and nobody else is heavily using the connection at the same time (downloads, streaming, router packed with devices), maybe you'll survive. But if it's your daily bread or you've got the whole crew at home using internet together, upgrading to fiber is way more hassle-free. Worth checking if it's already available in your area, because usually the price is pretty affordable these days.