Most components do not place restrictions on how many input values there are. They will run multiple times if more items or more lists are provided.
If you wish to limit a specific input to only have single item (or a single list) then there is nothing in the api that will do this for you, you have to add that check yourself and also record an error message.
I may have misunderstood your problem, but have a look at this discussion: C# About GH_ParamAccess type - #2 by DavidRutten