Here's the thing. Knowing that you cannot carry a knife for defense in the country where you live, you decide the best advise is to carry two knives. Now, if you're never stopped by the police, that's awesome. However if you are, for what ever minor reason, you now have to justify carrying two knives. "predominately for defense against animals" probably won't pass the sniff test with most police. Weapon laws are weird in many countries. In Canada, we are not allowed to carry anything for self defense. I can legally carry a broadsword down the street, but I should expect to get stopped and questioned. If I'm going to the HEMA club down the street, it is no problem. If I say I'm carrying it for defense, I get arrested.
Making broad statements about what someone should carry (or how many of an object they should carry) without taking into consideration the local laws is foolish.
If the requirement for doing damage was a knife had to be sharp, more thugs would carry scalpels. As it is, they seem more inclined to use gas station knives, which can't hold an edge, are made from cheap steel, etc., yet seem to cause massive damage when required
The self defence agaisnt animal tatement, i had canada in mind. as i had it from 2 secondary sources you can carry some items for self defence against animals. (pednign state, i dont know how much autonomy is there)
Going to have to break this down, but the simple solution is, carry a swiss army and use it when appopriate for defence. The defences given for the blades does not include self defence, but if you are lawfully carrying it or any object you can use it in defence of yourself if the force level is reasonable. If a police constable stops you and searches you (needs "reasonable grounds" to do so, if they dont need that in your country i suggest protesting hardcore) and demands to know why you have 1 or 2 swiss army knives, dont tell them any reason. You are not required to provide a reason for the stated knives that arent lockable up to 3 inches in length. If you arent required to tell them something, dont. You only dig yourself in a hole, if they arrest you etc, request legal aid or request that you contact your lawyer. (they have to provide it to you or let you contact yours)
As for defences, they dont cover self defence, nothing on the page mentions animals. And i keep getting directed to hunting weapons. But if you were hunting, you cant do that on public land, you can carry what ever really, so if a dog comes over and attacks you, you shouldnt have much of a issue, only justifying the force on it.
Second from last point: Other people have stated "look at your laws", i would presume that would be common sense before taking any advice anyway. Secondly General carry advice say "make sure you keep it sharp" or stuff like that, isnt related to laws, its just general knife ownership and use discussion.
Last point: The requirement for a knife to do damage or any object that cuts/thrusts is a small surface area. Im not being abolutist here, but sharpning a edge on a knife doesnt hurt it at all and if the design for thrust knife has a edge, make sure its sharp, it wont hurt it in the slightest and be better than if you left it blunt or chipped etc. Needles by their nature, you only have to make sure the point is pointy, by needle i mean spike bayonets and that design of rondel and things like it, things with no edge.
Addendum: some of the law disucssion is divided between the two posts. I also only hit the high notes for some laws, feel free to read the sources etc.
@Monkey Turned Wolf See below for some sources on it.
Sources:
Selling, buying and carrying knives ( hit the high notes)
Self-Defence and the Prevention of Crime | The Crown Prosecution Service
Dangerous Dog Offences | The Crown Prosecution Service
Police powers to stop and search: your rights (hit high notes for stop and search in passing)