Used judiciously Chimera can be reasonable, if unreliable, friend to the corp; used poorly it will do more work for the runner than for you. Costing two credits to rez and having the unique property of being any of the three main types of ICE you want it to be Chimera's purpose should be fairly obvious - it's there to rush out an agenda at some point between turns one and four. Assuming you're happy to leave your centrals open to attack you can score out an agenda with as little five credits in the bank - though I wouldn't recommend such a thing unless you're running a LOT of low cost operation economy. Of course doing so is fraught with risk - Criminals have Inside Jobs and Siphons, Shapers have SMCs, and every Anarch and their mother runs Faust. Okay, let's be honest; everyone's running Faust. Still that's the thrill of rushing. If you're not up for high risk-high reward games then it's not the deck archetype for you.
Chimera can also be a pain. If used to defend centrals or assets or indeed used any time after the first three to four turns it will hurt far, far more than it will help. At zero strength and with only one subroutine it will cost most runners just a single credit or card to pass. Meanwhile you're spending two to rez it every turn: the exchange is not in your favour.
How many copies of Chimera should you include in your deck? For the most part the answer is none. If you are putting it in you probably want three. For it to do work for you it needs to be seen early. Of course then you're going to be drawing and promptly discarding it in the late game when even a humble Quandry has more utility. Arguably simply including regular gear check ICE is the better option. If unmolested and invested in Ice Walls, NEXT Bronze and the like can even gain in utility as time goes on.* That said it's not always an either or choice: a Chimera often works best when paired with another gear checker. It takes a little more cash and a little more set-up but with a Quandry in front of it Chimera can take on a single SMC or an Inside Job.
*Though they often don't and - outside of certain specialty decks - investing in Ice Walls is rarely a good idea.