Thousands are buried every day. But many are buried in multiple casket graves. For example my parents are on top of each other in their plot. Here in the U.S. there is still wide open parts of land. It is getting crowded though. An increasing number are being cremated and this helps to slow down the expansion of cemeteries. Sometimes in Europe and parts of the United States, cemeteries are moved or simply built over. As to the second part of your questions. Metal caskets are designed to preserve the body for longer periods of time but eventually, after about 20 years or so, keeping in mind that there are thousands of variables that effect this, a body will rot and decay. The casket will last for a few centuries maybe a little longer but that too will break down. You can see this in older cemeteries with wooden coffins that have already rotted. The ground over the grave sinks down after the coffin breaks down. Today, many graves are lined with concrete boxes or vaults and the coffins are placed inside these. This causes a further delay in decomposition and deterioration. But ultimately we all end up returning to the earth regardless of what we are burtied in. Nothing can stop that.