Hey @e-koma, thanks for asking this! And welcome to the forum 
In general, testnet is a good place to stress test your application, but we do need to be careful to ensure that the network stays useable for everybody.
We don’t have exact TPS numbers that we can advise right now, but small tests of ~10 TPS should be totally fine on testnet.
If you want to perform a bigger test, please send us a message on Discord (you can tag @Flow Team in the #developers channel) and we can help you out. It’s also really helpful for us to know more about your tests so that we can capture a variety of network usage profiles!
Depending on the use case for your tests, you may also want to use the emulator to perform stress testing. You can configure the emulator with artificial block delay to simulate real network latency. Let me know if you’d like me to expand on that.