IOTA can theoretically run perfectly fine with just a single node.
However, in current stress tests, it seems like nodes can handle about 1000 TPS. So for more than that, there would need to be more nodes.
Keep in mind that currently, the coordinator limits the throughput of IOTA, so the number may vary from the above until it is changed.
To answer the question of security, a single node isn't secure. Security increases with the amount of nodes -- an attacker needs to be able to control 34% of the network in order to make a malicious attack, so the more nodes, the less possible this becomes. Security is therefore bound by how powerful any given attacker may be, divided by .34.