Is there an upper limit on pictures in library?

What kind of stuff impacts max_input_vars? Cached items?

> Is there an upper limit on pictures in library?

No, the limit is your server power and space

> What kind of stuff impacts max_input_vars?

Form elements on submitting a form as the error tells. Whenever your save something a form is submitted.

does this include cached items? or just page elements?

Is there someplace I can look to asses consumption so I need ot bump up numbers?

Cache items are not form elements… If you don't know what a form is:
https://developer.mozilla.org/en-US/docs/Web/HTML/Element/form