Try searching for something like "Software Timer"

The general principle is that you use a single (hardware) timer to generate a timebase or "tick";
On each "tick", the hardware timer updates each of the currently-running software timers;
When a software timer expires it could call a function and/or set flags to notify the main code loop.

Something like this:

unsigned int timer_1;   // Countdown value for Timer-1
unsigned int timer_2;   // Countdown value for Timer-2
unsigned int timer_3;   // Countdown value for Timer-3
                        // etc...

void timer_isr( void )   // Hardware timer interrupt handler
                         // use appropriate compiler-specific syntax!
{
   if( timer_1 )
   {  
      // The countdown is running
      if( --timer_1 == 0 )
      {
         // The countdown has reached zero; ie, the timer has expired
         // Set flags and/or call functions as appropriate
      }
   }

   if( timer_2 )
   {  
      // The countdown is running
      if( --timer_2 == 0 )
      {
         // The countdown has reached zero; ie, the timer has expired
         // Set flags and/or call functions as appropriate
      }
   }

   if( timer_3 )
   {  
      // The countdown is running
      if( --timer_3 == 0 )
      {
         // The countdown has reached zero; ie, the timer has expired
         // Set flags and/or call functions as appropriate
      }
   }

   // etc, etc,...
}

You start a timer by setting its countdown (global variable) to a suitable non-zero value.

Depending on how many timers you have, it may be more efficient to put them into an array, or even a linked-list, and have a loop to handle them in the ISR...

This is exactly the kind of technique that an RTOS would use for its timers!