How to declare a function with an argument which is an array?

Unlike other languages, MATLAB can accept matrices as a single argument; so you could just check that the input argument is a vector of length 32:

function x = myfunction(arg)
    if length(arg) ~= 32
        error('Must supply 32 arguments!');
    end

    %# your code here
end

If it's a variable number of arguments, check out varargin:

function x = myfunction(varargin)

But for 32 arguments, consider using an input structure:

function x = myfunction(argStruct)

    if length(fieldnames(argStruct)) ~= 32
        error('not enough arguments!');
    end

Supply arguments in the structure, then pass the structure:

>> myArgs = struct();
>> myArgs.arg1 = 5;
>> myArgs.arg2 = 7;
>> %#(etc)
>> x = myfunction(myArgs);

Then in the function, you could either call argStruct.arg1, etc, directly; or unpack it into 32 different variables inside the function. I would give the fields descriptive names so you don't refer to them as arg1, etc inside your function. For that many input arguments, people using the function probably won't remember the order in which your function requires them to pass inputs to. Doing it with a struct lets users pass in arguments without needing to think about what order those inputs are defined.