If you have more than one task you want to run at startup, you could include code for each task in a single stored procedure. But that’s generally bad coding practice. Go with the modular approach and create a separate stored procedure for each distinct task. With sp_procoption, you can set more than one stored procedure for automatic execution. The MSDN documentation was not clear regarding the order of execution, though. I thought there might be something similar to sp_settriggerorder, but I wasn’t able to find anything like that. I ran a test on SQL 2014 with 3 stored procedures set for automatic execution.
Dave has interesting notes on procedure run order, where these procedures need to live, and even some ideas on what you might put into startup stored procedures.