What real life OpenPGP public key has the biggest amount of signatures on it? Have you ever seen key with dozens or 100+ signatures? Link to it will be very much appreciated.

I have read that old Zimmermann's key had a lot of signatures on it, but I was not able to find it. In fact when I search MIT key server I am not able to search for 'Zimmermann' name at all for some reason.
Over time the idea of what is a strong enough key has changed and keyservers have removed keys they consider unacceptablly weak. IIRC zimmerman moved on from pgp a long time ago and I expect all his keys have been removed from keyservers due to being weak. 

I was able to find a key with 230 signatures just by educated guesswork. https://pgp.cs.uu.nl/stats/587979573442684e.html . Unfortunately while that site shows data on individual keys it doesn't seem to have any ability to search/sort keys.

Data on the relationships between all keys in the "strong set" is available in "Wot files" the WoT files can be downsloaded from https://pgp.cs.uu.nl/archive/wot-0.3/ , software for working with them is available at https://pgp.cs.uu.nl/wotsap/wotsap.html . I imagine if you really want an answer to which key has the most signatures it should be possible to write code to extract it from the WoT file.

I would note that it's easy to generate a bunch of keys with random name and cross-sign them with your main key. Number of signatures on a key alone is not really a good indication of anything useful.