Google Cloud Text-to-Speech: Monthly Free Characters (4 Million for Standard and WaveNet Each)
Google Cloud
Free synthesis each month: the first 4 million characters each for Standard and WaveNet voices, and the first 1 million characters each for Neural2, Studio, Chirp 3: HD and Polyglot
From the "Free usage limit" column of the official pricing page: WaveNet "0 to 4 million characters", Standard "0 to 4 million characters", Studio "0 to 1 million characters", Neural2 "0 to 1 million characters", Chirp 3: HD "0 to 1 million characters", Polyglot (Preview) "0 to 1 million characters". Counted separately per voice type and reset monthly. Google notes that billing must be enabled to use Text-to-Speech and that you are charged automatically once you exceed the free characters for the month.
You need a Google Cloud project with a billing account (credit card) enabled; usage beyond the free characters is charged automatically, so set a budget alert and quota cap. Users in mainland China need a proxy. Free characters are counted separately per voice type, not as one shared pool.