With FastOTP you can customise the character count of the tokens being generate, from a minimum of 4 characters to a maximum of 10 characters 
curl --request POST \
  --url https://api.fastotp.co/generate \
  --header 'x-api-key: fastotp_xxx' \
  --header 'Content-Type: application/json' \
  --data '{
	"token_length": 6
}'
The OTP generated will be have 6 characters
To see this in action, make a POST request to the /generate endpoint. You can read about all the parameters you can send in the API Reference.