Alright, sice it's so quiet here lately, I had to find a workaround - it's a bit dirty, but it works.
Here's what I've done: I made 'strategic' fields register into manually crated table X in my database (earlier the form was fully email-results-only) while submitting. Then I placed this form in a module. Then in native Joomla article I used Sourcerer (great extension!) and wrote a code to first count submitted forms in the table X and then simple condition to show the module with form inside this article only if the amount of submissions is lower or equals N, otherwise it shows that 'Sorry' text.
I'm still wondering (out of ineterst), what could be the more elegant, pure-Seblod method though...