i can't find any information inside the w3c html strict spec http://www.w3.org/TR/html4/sgml/dtd.html
PHP USERS:
If you are using php to process your form take note that php has a max_input_vars setting in the php ini file. I believe the default is 1000.
I ran into this while building a cakephp app. To see if this is affecting your post, count how many rows of data you are putting into the form and then count how many rows are actually posted back.
So essentially your form would be limited by the number of input vars you have.
This isn't really an direct answer to the OP but I think it would be helpful for anyone searching for a limit on input fields.