Hi Aneeta,
The right amount to feed per day always depends on calorie needs. In senior cats this can vary a lot. Some have very little age-related disease occurring and are just as active and healthy at 11 as they were at 6. Other cats who are not as active may have reduced caloric needs. Others who are more elderly may have increased needs to maintain their body condition. This is why there is no standard nutrition for senior cats and must be catered to their needs. Your vet can help you the most with understanding what those needs are and the best nutritional support.
In general, most average 10lb cats living indoors should get about 250 calories per day. To find out feeding amounts, you take the calories needed per day and divide it by the calories on the food package label, near the ingredients list.
Taking Fancy Feast as an example (only because I know their little 3oz cans range from 80-100kcal) with an average of 90kcal per can, an average 10lb cat requring 250kcal per day would need to eat about 3 cans per day (250 divided by 90).
For more help, you can go to our weight calculator that helps you assess your cat’s body condition and feeding amonts based on the calories per package of the food you have their age, etc. Here is the weight calculator link: https://cats.com/weight-calculator
I hope that helps!