This question is too broad. It depends on the currency's system. And the figures are rather arbitrary.

Let's focus on bitcoin's proof of work system. Processing power (of miners) is required here to confirm transactions. Protocol adjusts processing power needed to confirm transactions depending on the current processing power.

If total processing power is low it is easy for attacker to enter the network with large processing power (comparable to total current processing power) and execute attack.

So it is important to have huge processing power in the network and that it is as much as possible distributed among the miners.

What is considered huge is arbitrary. What is considered sufficiently distributed is also arbitrary. Currently in bitcoin you have large mining pools that you can considered each a single miner. Don't know how many of these together at the moment have processing power comparable to all other miners.

If 10 mining pools have over 50% of network processing power they can (in some scenarios) collaborate and execute attacks.

Thank you very much for the answer. So you think a matrix of scale and difficulty rather than just scale is a right measure. – Slowblogger – 2017-12-05T06:10:52.457