Aggregating high volumes of data from many MySQL servers
Yes, MySQL will be able to do 20 million inserts per day, no problem. My calculator says 20 million per day translates to roughly 231 inserts per second. See e.g. this blog post from Percona's Database Performance Blog from 2010 which talks about more than 36 thousand writes per second. With the optimisations that have gone into MySQL since then as well as the hardware improvements, this number will obviously be even higher today.

And from a skills perspective, since you're already using MySQL, it might make sense to use that for this aggregation database as well.

Note that you can increase write throughput by combining multiple inserts together in a single transaction.

However, you may be able to use multi-source replication:


  MySQL Multi-Source Replication enables a replication slave to receive
  transactions from multiple sources simultaneously. Multi-source
  replication can be used to back up multiple servers to a single
  server, to merge table shards, and consolidate data from multiple
  servers to a single server. Multi-source replication does not
  implement any conflict detection or resolution when applying the
  transactions, and those tasks are left to the application if required.


So if you're able to use this, then you won't have to write any code to do the actual replication.