Hi h r,
I'm not sure what you mean by "expression number".
Are you trying to recursively define a function "In", so that
In[i] depends on In[i-1] ?
Or are you trying to define a List "In", so that
In[[i]] depends on In[[i-1]] ?
Note the syntax difference of single bracket "[" vs double bracket "[[". The latter refers to Part, and is defined here:
https://reference.wolfram.com/language/ref/Part.html