Disclaimer - This is basically my opinion, not any kind of rules.
So what I like to do is the least amount of animation needed to get the effect I wanted. I find that in engine blending helps a lot with this and without it, it’ll be a nightmare and super time consuming. So I do use a lot of blending in stead of transition animations. For higher quality you’ll need to do more animation work.
The way I generally do it is if I have an animation I’m gonna blend into, I start the animation with about 5 frames of almost static pose and then go into the move. So take a sword attack for example, I start at the wind up (not idle), hold for about 5 frames, and then do the move. So in game when I blend to that move I have enough room to do a good blend before the move even starts. Moves like that is generally so fast that you probably don’t need any extra transition animation regardless. I also leave more frames at the tail end for blending purpose.
When the move is complete you blend back to idle, or running, or whatever is looping at that point. now with blending back there is a possible need for a transition animation. Currently I don’t use any, anywhere, but you quickly see where you might need some. My theory there is you kinda plug in a transition only, so you blend from the end of your move, to a transition, then to idle so it’s a double blend, but can be short blends. The reason for this is so you can recycle the transition animation, since many moves will end or recover the same way.
But anyway, I’d say start testing it out for your own purpose, do some moves put them in your game and start to look at what you feel you need to get the look and quality you’re after. The only thing really to remember is that there are no rules there specifically. Animation blending is awesome and useful so make use of it as much as you can, it’s a real time saver not to have to think about every little transition. It’s probably generally better to have less animations rather than more, but you really need to have loads and loads for it to become a technical issue, so you’re pretty open to do what you want pretty much.