With each recharge cycle, a rechargeable battery will lose a tiny amount of its total capacity. An iPad battery is rated to retain ~80% of its original capacity after 1000 full recharge cycles - a full recharge cycle being any combination of charging that together sums to 100%.
While there are many variables that can affect the calculation, As simple math suggests that your iPad will lose about 1% of its capacity for every 50 recharge cycles. When performing a system software update, the measurement algorithm may be refined, or the battery health recalibrated.
You can can check your battery cycle count in iPad settings:
Settings > Battery > Battery Health
More information:
About iPad battery health and history on iPad Pro (M4) models and iPad Air (M2) models – Apple Support
You can tips and advise for maintaining your iPad's battery and its health here:
Batteries - Maximizing Performance - Apple