There is a minimum bet equal to the big blind like in no-limit poker and a maximum bet that is equal to the pot (all the chips coming from bets and raises until that moment).
For example:
The pot is €100. The next round, one player bets €20, and the next two players call this €20 bet. In case the fourth player intends to raise, the maximum would be €180. This raise matches the €100 in the pot, the first €20 bet, the two additional €20 calls and the player’s own €20 call. Such a game helps improving your maths abilities too!