The simplest form of service chain can be one SCN. This tutorial illustrates 4-node service chain, however, you can set up a single-node service chain as well. Simply pass
--cn-num 1 instead of
4 to homi in 'Step 1:Create genesis.json and nodekeys'.
The minimum number of SCNs to achieve high availability under BFT algorithm is 4. Having 2 SCN nodes is not enough, because if one SCN fails, the other one can not reach a consensus.