Is there any rule for charging the battery for the first time?

There is no hard and fast rule. I generally charge my iPhone to 100% the first 3 or 4 charges and then adjust to the recommended optimum charging. Batteries and there charging algorithms have to learn what 100% is and if the user doesn’t do it, the optimized charging algorithm will automatically charge it to 100%.

>>If you have Charge Limit set to less than 100 percent, your iPhone will occasionally charge to 100 percent to maintain accurate battery state-of-charge estimates.<<

About Charge Limit and Optimized Battery Charging on iPhone - Apple Support

The iPhone has to do this so it can calibrate to 100% battery capacity. It’s normal. 

Charge as needed. See this for additional information on handling batteries on Apple devices. Batteries - Maximizing Performance - Apple