Heat can be a major problem for cabinets and servers. You need to make sure the data center keeps the overall temperature low enough with good air circulation and that the rack itself has good exhaust fans and ventilation.
Standard rack sizes are 40-42U. It's not wise to fully pack a rack as the heat does become a problem. Most often the limiting factor on how many servers can be placed in a rack is due to the electrical power assigned to the rack. Many racks only have 15-20 amps of power which is going to limit the total number of servers regardless of how much physical space there is.
Generally don't recommend closed cabinets. Then the cooling all depends on your overall solution, not really determined by number of servers per rack. In places such as Equinix you should have no problem filling a full rack with 1u servers.
Even still I dont reccomend filling a rack up. I would always leave some space.
Heat control is very important in general as you probably know the servers run best at a certain temperature. That is why most DCs will allow you to see the temperaure of the core parts of your server.
Heating issues on cabinets
