Not necessarily. Its certainly possible to run MongoDB on a machine with a small amount of free RAM.
MongoDB automatically uses all free memory on the machine as its cache. System resource monitors show that MongoDB uses a lot of memory, but its usage is dynamic. If another process suddenly needs half the servers RAM, MongoDB will yield cached memory to the other process.
Technically, the operating systems virtual memory subsystem manages MongoDBs memory. This means that MongoDB will use as much free memory as it can, swapping to disk as needed. Deployments with enough memory to fit the applications working data set in RAM will achieve the best performance.
With WiredTiger, MongoDB utilizes both filesystem cache and WiredTiger cache. By default, the WiredTiger cache will use either 1GB or half of the installed physical RAM, whichever is larger.
MongoDB also automatically uses all free memory on the machine via the filesystem cache (data in the filesystem cache is compressed).