Parsing binary data into multiple ctypes Structures

So I do have a binary file, that i want to parse. In this file I have a header and the header defines how the rest of the file is structured.

For example the file would look like this (as a mock up):

Version_Number = 001

File_Type = config

Number_of_struct_A = 4

Number_of_struct_B = 3

struct_A

struct_A

struct_A

struct_A

struct_B

struct_B

struct_B

And so far I am able to read the first structure in my file, as shown in this example: Parsing binary data into ctypes Structure object via readinto()

So for example I will have the following types of structures:

class FileInfo(Structure):
    _fields_ = [("Versions_Nr", c_byte),
                ("File_Typ", c_byte),
                ("Number_of_struct_A", c_byte),
                ("Number_of_struct_B", c_byte)]

class Struct_A(Structure):
    _fields_ = [("A", c_byte),
                ("B", c_byte),
                ]

class Struct_B(Structure):
    _fields_ = [("C", c_byte),
                ("D", c_byte),
                ]

#read in my file
file = open('test.cfg', 'rb')
header = FileInfo()
file.readinto(header)

print(header.Number_of_struct_A)

So this will give me the header, but i have no idea how I should save the data from my file, so that i can use the information about the number of structures, that i just gained. Or for that matter, how to store the data at all, as reading the file over and over again doesn't seem to be a good option.

I hope my question is understandable.