This is a typical market sizing question. You should clarify if the goal is to find only the number of sim cards sold or value from the number of sim cards sold. You can answer by the following approach
1. Start with the population of Saudi Arabia
2. Segment the population by age (0-15yrs, 15-30, 30-45,45-60,60+)
3. Make assumptions on which segments will need a sim (e.g. 15+), how many devices (one for phone, one for tablet etc.)
4. Make assumptions on how many in the segment will need a new sim (either additional device, or get a new one). Multiply these percentages to the segment volumes.
5. Add the numbers to get the total number of sim cards sold.
If the goal is to find a $ value from the sim cards sold then multiply the number with an average cost of a sim.