My HTC One (Android 4.3) told me that I can only have 10 alarm clocks at maximum and that I should delete one before adding a new one. Why, and is it possible to have more than ten?
Get Timely Alarm Clock. I've never tried more than 10 alarms but I guess it should be worth a try. It's free after all & extremely sexy.
Also you should try adding calender events instead of alarms and change the sound to something loud.
Is it possible to have more than ten (alarms)?
Maybe not with HTC clock, but yes it is possible.
As mentioned by dotVezz in the comment, "it's definitely not a constraint of the OS".
Android doesn't set any restrictions regarding how many alarms they can store/handle, even on per-app basis. A counter-example to OP's case: a stock clock on Nexus 4 (Android 5.0) can have 15+ alarms (I stopped adding after 15 alarms).
If you want to have more than 10 alarms, then you can just search it since "there are a ton of alarm clock apps in the Play Store." As already mentioned in other answer, Timely Alarm Clock supports more than 10 alarms.
Why (is it impossible to have more than ten alarms with HTC clock)?
I can't really answer this without speculating as this is a decision made by HTC. Some possibilities that I could think off:
- Less alarms are easier to manage
- Less alarms won't hit the OS performance.
But for the technical part, Android is open-source and anyone can customize it as they like, including adding the alarm restriction on the clock app.