According to Luke Dashjr's stats there are currently around 61.000 bitcoin nodes supporting the bitcoin network. https://luke.dashjr.org/programs/bitcoin/files/charts/historical.html

What if an attacker sets up around 100.000 new bitcoin full nodes around the globe (at 100$ each this could be feasible with just 10 million $ invested in total), and at certain time in future rolls out changes on all the nodes in his possession, which modifies some rules? His modification could influence the validity of a certain blocks and its transactions - this would likely lead to being blocked soon - but he could be also just filtering out transactions for certain addresses, or not propagating blocks from certain miners / not relaying them to other nodes. The rest of the honest nodes (in this case 37%) would still relay transactions and blocks in full scope. How could such attacker hurt the bitcoin network the most? Note that the attacker is not a miner, and he is not controlling any hashing power, he just controls the majority of existing full nodes in the network.