4500

You can keep placing objects. There is an actual limit, but it's not 4500. I'm not sure what it is.

If it's causing lag, maybe see if you have any hidden objects you can store or use a large hedge where you have two small hedges.

The limit is 7500. You'll be fine but you'll receive warnings periodically until you receive the last one which says you can't place anything else (@ 7500 items).

As soon as I hit 4500 my game started crashing seconds after loading, it happened on multiple devices so I had to click edit mode as soon as it would load (crashes could be as quick as 2 seconds) I stored a bunch of random stuff and that seemed to fix it.