As a general rule, the sheet modules and the ThisWorkbook module should contain only event procedures, and nothing else.
All other code should reside in regular code modules.
Some would argue convincingly that private procedures used only by the event procedures in a module should be contained in the same sheet/thisworkbook module in which they are used.
There is no practical limit to the number of modules allowed in a project, so you should organize your code into multiple modules, based on the type of functionality provided by the code.