I recognized there's a limit on the K2-Menu-Option "Latest items from (one or more) users or categories" somewhere around 500: minecraft-heads.com/heads/alphabet/overview

What if I would like to show more? If I use a number higher then 500 the Website got blank and no content is shown.

This is probably a memory related limit.
You can enable error reporting in your site's global configuration and see the actual issue.

I would advise you as a professional that 500 items are too much, and apart from the page being too intensive for the server, the loading time will severely affect your Google ranking.

I didnt know the speed has affect on google-rank, good to know.

I already limited Bots to the startpage with index/nofollow, so google etc shouldnt detect this, or is my logic wrong?

I think yes, blocking your homepage with nofollow means that the googlebot will not follow any links towards items or categories (which is really REALLY bad).

Even if you don't index your homepage it will be really frustrating for users to wait so much time for the page to load. 500+ images (requests) might also cause browsers on underpowered smartphones to crashe

Oh actually those images are very small. In my tests the website loaded in 12-15 seconds. That's not smartphone-compatible, but that's not the goal of the website anyways.

But never the less - I know you're right. Long loading is bad, but this overview is usefull for people who know what to do with it and I dont see another solution. Is it possible not to let google go to these special overview-sites? If not I think I've to limit to 100 items or so.

Hi razen,

"Is it possible not to let google go to these special overview-sites?" What i know wants Google to index Pages which Users get. I wouldnt try Tricks like this (which i dont know how to do).

Regards

12 - 15 secs is way too long. The ideal scenario would be 3 seconds and less. I would cut down on the number of items. Furthermore as a user I would hate to try to find an interesting item amongst hundreds of others.