MySQL nested queries
I think this query does what you are asking. The sub select gets the counts for each id. I then joined it back to t1. The outer query gets the max for each id. I didn't put the WHERE clause since I don't know the criteria but you can easily add it.

Here's a link to SQL Fiddle

select id, 
  class, 
  counts, 
  max(numOcc) as Occ
from t1
left join 
(SELECT event_id, reconstruction_algorithm, count(1) as numOcc from t2 
  group by event_id, reconstruction_algorithm) a
on t1.id = a.event_id
group by t1.id, t1.class, t1.counts