How many ethereum clients must adopt a hard fork so that it will happen without creating two chains? Probably a 50/50 splitt would create two chains and we have two Ethereums?
Tehcnically, there is no a minimum number: if just a 3% of the network opt to not adopt the hard fork, they can technically set up their own network, send transactions and deploy contracts just using the ethereum protocol: just they can't interact with hard-fork adopters nodes.
However, is expected that, if a small pool of nodes does not accept the hard fork, they later switch to agree on it, since its own network ether value would drop a lot in value. But, even a 70/30 split or 80/20 could perfectly lead to a chain split (in fact, a network worth 20% of current ethereum would be bigger than several crypto networks).
Knowing this, calculating a minimum % of agreement to prevent the split is complicated, and depends a lot on economy, game theory and considerations of various types. If i.e 20% of the network choose to not hard fork, they could consider that in the long run its network is more valuable and trustworthy since they did save blockchain immutability and 'code is law' principles (in fact, that would be the main reasons for nodes which choose not to adopt the hard fork). Then they should to choose an alt name and set up some official organization and dev team.
That is a possible scenario. Or maybe, after a close 58/42 outcome, it would happen that several nodes of the smaller network choose to adopt the winner chain because of that outcome, and that then leads to the smaller chain to dissapear proggressively into the bigger.
So its really difficult to predict, but, from an exclusively technical point of view, just 3 miners and 10 nodes would count as another chain, even if that chain would have really ultra little traffic and market capitalization.