How can we show Captcha using Sitecore Forms and validate?
It doesn't exist such a field out of the box.
@Bart Verdonck created a module : https://github.com/bartverdonck/Sitecore-Forms-Extensions which has a control to support google reCaptcha.
You can download it from : https://github.com/bartverdonck/Sitecore-Forms-Extensions/tree/master/downloads
He has a series of blogposts about his module: