hello greetings to all …
the information that I personally have found is not concrete enough for me (I have not been smart-enabled) …
How many messages per day can I send to TTS if my device is class A? 30? 96? 864?
How many??
It will depend on message (payload) size, data type(*), LoRa Spreading factor for the given messages (SF) etc. Use one of the available airtime calculators to estimate your application/instance using the TTN FUP of up to 30 seconds airtime per device for uplnks (downlinks are a different kettle of fish and should be avoided where possible due to community impact). This is for TTS(CE) aka TTN V3, a private instance may have different rules - talk to TTI team to see what may be allowed, but will in any instance need to stay within local legal limits (Duty Cycle, Dwell-Time, LBT, etc.)
(*)Use bits and bytes, use compression schemes, utilise available FPorts as identifiers etc to be air time efficient (Forum search - e.g. working with Bytes), avoid float and never use ASCII Text in payloads! 
)
Depends on size of uplink, here’s some concrete: