I need a little digital io for my project since cortex-m is not economical for me I need to know can i use Cortex-a to do some embedded jobs?
20-30 timers are already quite numerous. Luckily, the number of timers that can be supported by a real-time OS might be limited only by the available resources in the processor. The processing speed imposed depends on the task associated with each timer. Cortex-As have the speed to satisfy your requirement, and, as you stated, you can get more processing power per cost on Cortex-As. Your next step is to evaluate the RTOS for your project.
Yep it does sound like a lot of timers and I guess external clocks too. So it really depends on how much they each have to do and how quickly, but also you'll have storage requirements for all the routines and their data. Are you hoping to use the controller as a low level sequencer for most operations rather than get signals back as each job is done? That's a common sort of job for Cortex M microprocessors but a Cortex A series gives you extra speed and size.