When -> I set up one RND in the lowest wage city as soon as I get a decent money flow. Around 5-6mil a month. usually after 4-5 profiting retail stores (all my retail stores at the beginning are seaports reliant).
How many-> goes like this - each RND research 2 subject (5 tiles one and another one with 4 tiles). all have *some* minimal training budget, but a fairly low one (usually RND costs are around 350-380k a month per RND with the training involved).
The 5 tiles research do it as a 1 year duration research and the 4 tiles as a 2 years duration. after 2 years I set a new RND with the same method. sometimes I like to research 6 tiles one subject and 3 tiles a semi product research (plastic, aluminum etc...).
That way every year I get to ship a new line of product into the market. If competition exists I will use several methods to deny it:
One I'll research MORE than the competitor.
Second, if it's a small company I'll try to deny profits from them by selling their products cheaper to deny profits, that way I'll slow down their expansion.

Research line - depend. A. on my expertise. I'm good with toys? Racing car will be a first one as long as I can get the materials to start manufacture. B. what's good in the market. I really like Cola as it tend to be really profiting and easy to manufacture. So as long as the market can be easlly accessed, I will usually start with Cola.
From there on I'll go with whatever fits my availability to manufacture - I'm doin cold pills? why not research all the drugs section and after that go to manufacturer perfume and other chemicals based products? That what I will do.
Think I'm wrong? have a better way to dominate the market? just have a different way or another tip for us capitalism player?
Please, share with us