My inline fuse burned and I don't know how many ampere the new fuse should have.
Couple things to note. First one is that as @Paulster2 mentioned, the fuse itself should have its rating printed on it.
Second one is that if your fuse blew, there's a reason for that, and you really should try to figure out what that reason is before you start putting fuses back in. The Anet A8 isn't exactly what I'd call a high-quality kit, so there's a chance you've got a blown FET or something somewhere that has shorted closed and is just drawing power nonstop. You should start by getting a multimeter and checking the continuity of the power traces for your hotend and bed, without power in the system. You can do some basic safety checks that way.
Once you've gotten that done, if you're referring to a 12 V feed line, that should be somewhere in the ballpark of 20 A fuse. If it's a fuse on the mains power, it'll be like 1 A or 2 A depending on whether you're on 240 V or 120 V mains (respectively).