My knowledge on other fighting games isnt as extensive as dragontamers so my reply will be limited to Smash 4 vs Brawl
In Brawl, buffering was a base 10F at the end of an action during which multiple inputs could be done, so their corresponding actions would begin execution as soon as the ending lag of the previous move finished. So for example, for a move that ran for 20F total (including ending lag), the "buffer window" would open up ON F10 and close ON F20. Anything that was buffered would begin on F21. Buffered actions were executed in their order of input, and were executed normally; buffering short hop then a neutral attack would result in a SHNAir, with the NAir being executed as soon as possible. Inputting, say, Gyro charge then shield would result on the Gyrp Charge being cancelled as soon as possible. To be honest, I don't remember how jump cancelling interacted with buffering (would buffered short hop then an upwards attack yield a SHUAir, or a JCUsmash? Idk)
I said "base 10F" because for actions that ran for less than 10F, the buffer window would be cut short. For example, in Brawl, item tosses were 6F I believe. This means that the buffer window would open on F1, close F6, and actions would begin F7.
As for how many actions could be buffered, I'm not aware that there was a limit, but if there was, it was at least three. Falco's BDACUS (buffered DACUS) required three inputs to be buffered: dash, dash attack, then USmash.
In Smash 4, buffering works differently. i don't know if the buffering window has changed, but I do know that you cannot buffer mutliple actions and have them all be executed. Instead, only the last action inputted will be executed. Thus, buffering a short hop then a neutral attack will not result in a SHNAir: it will only result in a jab.
In both games, buffering is not limited to attacks; you can buffer out of nearly anything that restricts your ability to execute things. This means you can buffer out of jumpsquat, airdodges, spotdodges, shield hitlag, shield drop, landing from an empty hop, catching an item, etc.