Each map has a budget. The budget is usually several hundred dollars, you buy objects using the budget. Or did you mean the actual limit, like if you were using a Budget Glitched map?
I believe I hit 373 items on foundry... yep, thats it, you can reach it by going with small items, or with a budget glitched map
Actually It depends. On various maps I've made hitting the item limit was common, even sometimes at about 50 items. One solution to this is to make sure you never save your map when you are currently hitting the item limit. One way to keep forging and avoid the item limit is to watch out for the item limit message, and when you do get it, save changes, end the game. When you start the game back up it should let you place items again. Good luck.