We use https://ui-avatars.com to generate avatars. You can pick length of initials, background color, font color and avatar size. It’s free and without limits.
We do not track usage, and there is no limits. It’s free to use and everything is done to assure fast responses with least delay.
Only terms are that we (the API) is not liable, and you agree to typical MIT licensing. Please use the service with respect, considering that some people depend on it, and it’s limitless and free. Be nice, that’s the only terms of usage.