I remember in Super Smash Bros Brawl, the game limited you on how much stuff you could put down in the maps you made using the map builder. Is that still a thing in the Super Smash Bros 4? I have yet to get it (or a Wii U) on the Wii U, and only have it on 3DS.
Yes, there is a limit, which depends on the size of the pieces.
You can see the limit represented as the "weight" bar on the bottom of the editor:
There is a limit, sadly, but Nintendo said they will update it so you can play other people's levels.
As for the newest SSB, there is only a few things and items to use at the moment which sucks.
SSB Brawl I heard the stage builder was better, with more stuff and also I think the limit was bigger.
The limit is kinda small for SSB4 at the moment.
Nintendo may improve this in the future though.