Is it possible allowed to send multiple transactions to deposit address generated by chipmixer?
Yes.
Or should it be one transaction?
No. But it's better for privacy to deposit to different sessions, and get a
voucher to combine an your old session with your new session.
There's no answer in FAQ
It's in there, although slightly hidden:
Since we don't have chips lower than 0.001 BTC, your deposit will be treated as donation. Be aware that if you send 0.0015 BTC, you will receive one 0.001 BTC chip. Sending 0.0015 BTC twice will result in two 0.001 BTC chips.