Why is that my maximum battery capacity drops and how best I counter that . BTW am using iPhone 11

Protect & Prolong life of your iPhone's Battery:

1. About the battery usage on your iPhone, iPad, and iPod touch ...
2. Batteries - Maximising Performance - Apple (IN)
3. iPhone Battery and Performance

Try some of these options too.

1. Avoid Screen Mirroring
2. Put on Low Power Mode
3. Put Auto-Lock on (1 min):- with low power mode it will be 30 Sec
4. Put Auto Answer Off
5. Disable Push notifications for apps you do not use often
6. Keep your iPhone Facedown to prevent notifications to light up the screen
7. Enable Auto-Brightness:- Settings > General > Accessibility > Display Accommodations > Auto-Brightness [Enable]
8. Manually lower brightness from Control Center
9. If you Want to watch continuous Video. Stream it on TV or a Monitor
10. Don't force close apps; Apps which are not on frontend screen are paused by the system to save energy. Apps require more energy to re-launch. Apps which are open in the background use large memory is a myth and quitting them is counterproductive
11. Location services to be allowed only while using the app
12. Set your iPhone to fetch email and calendar data from internet less frequently
13. Turning off Background App Refresh 
14. Charge before it drops below 20-30%
15. Try connecting to trusted Wifi wherever available instead of Cellular
16. Enable Airplane mode in areas with bad cellular and wifi coverage. Your phone keeps trying for reconnection with the missing/weak network. This uses a lot of energy
17. Switch off Wifi & Bluetooth while in Roaming or Not in Range
18. Enable "Wifi Assist" : Settings > Cellular or Settings > Mobile Data > Wi-Fi Assist [Enable]
19. Periodically Check the Battery Health (Settings > Battery > Battery Health)

iPhone 11 and later models, battery performance --> About the battery and performance of iPhone 11 and later

vishnuvardhan224 wrote:

Why is that my maximum battery capacity drops and how best I counter that . BTW am using iPhone 11

Because all batteries lose capacity over time. That’s why Pep Boys auto supply allocates about half their store space to replacement battery inventory.

Battery capacity is determined primarily by how many times you charge it fully (0-100%, or a combination that adds up to 100% such as 40% to 90% twice). It should maintain at least 80% capacity through 500 full charge cycles, which averages out to a 1% drop for every 25 full charge cycles (although it isn’t strictly linear).

Thus, one strategy to preserve capacity is to enable Optimize Battery Charging, then charge the phone overnight, every night. When the phone reaches 80% it stops charging, and all of the overnight phone activity will use line power instead of battery power, resulting in fewer charge cycles over time. It will resume to reach 100% around the time you remove it from the charger as it “learns” your usage pattern.