See the first tutorial.
https://mtk.sciml.ai/dev/tutorials/ode_modeling/
As @contradict mentioned, the solution only solves for a subset of the system, but using the symbols of the observed variables, such as sol[x_128], will retrieve their values. You can use the symbol in place of an index for this feature and it will compute their outputs on demand. Likewise, if you built this from a symbolic vector, sol[x] should work and return the timeseries for the full x, or similarly sol[x,i] etc. using the symbol x itself.