It depends on what you mean by "control the network". All that an attacker can do with a lot of nodes is conduct a sybil attack on other nodes. They cannot change the blockchain without any mining power, they cannot make coins move that they don't have the private keys for. All an attacker can do is feed bad information to a node and keep that node from the rest of the network.

The intention of my post came from the possibility to log nodes into a database and perform a exploit attack on that database (for example armitage psql db) . In some "worst case" this would effect a majority of the logged nodes.

So my question wasnt specific enough : "How much in % of the online connected nodes to the network need to be remotely controlled to expose the network to any kind of attack"

Thx for replies anyways, just trying to think like an attacker - i could log nodes and exploit them, right - how much would be too much for the network to become fragile.
@ Decoded, why arent nodes enough (im not a UBER programmer, just curious)

Thx for your input, shed some light on it for me.