You have 50 slots for buddies by default. If you add more than 50 buddies then you need to
buy more buddy slots from your ManiaPlanet Player Page. They cost 150 planets per slot. Once you have bought the slot, then you can add or remove buddies from those slots. It just gives you more room for more buddies.
If you don't add more slots and you keep adding buddies past your limit, it will pop up in-game and tell you that you've went over and to act quickly or Nadeo will remove some of your buddies. Presumably some of the longest inactive ones (I would hope).
As I said, the default limit in ManiaPlanet is 50 slots. In TMUF it was 64 slots (and 100 coppers per slot to buy more). In TMNF it's 16 slots I think.
The ability to buy buddy slots wasn't added until around 2009 or 2010.
If you play a lot online and are chatty with lots of people... you'll find yourself running out of slots pretty quickly I think.