BSF, I did the same thing 2 years ago. Bought 11 2-4 year old pairs and still have all 11 to this day. The first year I didn't retain any heifers because of the $ that cattle were selling for at the time. In 2015 I retained 3 heifers that are currently running with a purebred Gelbvieh bull, which will give me 14 fall calvers in 2017. My 2016 crop yielded 5 bull calves & 6 heifer calves which I'll probably retain a minimum of 4 for breeding. If you keep retaining your own heifers your herd will multiple relatively fast. The key (IMO) is using quality bulls. Good luck, after I got back into the business I'm wondering why I hadn't done it years ago.
I will add, cattle are addictive. Just 2 weeks ago I bought 26 bred heifers that were AI bred to an industry leading registered Gelbvieh bull for March delivery. That'll bring my herd up to 40 total in breeding stock. I'll probably forgo some cash crop acres to produce additional forage for a winter feed stock pile. The cattle work seems to fit into the schedule of my off farm job better than the grain farming does.
FWIW, in 2015 my son who was in middle school didn't want me to pay him for working for me all summer, he wanted to start his own cow herd so I bought him a registered cow/calf pair (which turned out to be a 3-n-1), a heifer calf by her side. He retained the heifer and is breeding it along side mine, and his cow dropped another heifer calf in 2016. He's now got 3, 2 will calve in 2017. I'm planning on buying a registered bull in the spring to keep his registered herd growing and maybe get into registered myself. Who am I to hold him back.
He's falling behind on his end of the feed bill since he hasn't had a steer calf to sell and he claims he's broke. 
I don't mind, he makes up for it in labor as he tends to our herd while I'm at the off farm job.
