I have a few questions to ask
1) Where did you purchase these heifers (local barn, order buyer, producer, etc)
2) How long ago did you purchase these heifers
3) What breed are they(purebred or commercial xx)
4) Do you have at least a rudimentary set of corrals / chute?
I think you will have a higher degree of success (fun and profit) if you start out "under-stocked". Presently, I feel you are over-stocked. Pushing any system to its maximum potential is a fast track to a breakdown. A cow herd is no exception. Over-stocking can lead to increased disease, parasitism, and decreased food availability. Feed (sack, or hay) is probably the most costly input cost of a cattle operation. The beauty of grazing cattle is that they utilize grass that is growing on your land. The less feed you have to supplement, the better. Keeping your stocking rate low will help ensure you have enough grass for the heifers that you are grazing (summer). Obviously, during the winter, you will need to supplement feed regardless how few heifers you keep. During this period, its hard to beat good fertilized hay and breeder cubes (21%). Regarding a bull vs AI: AI is more labor intensive and costly than many realize. You are either going to have to provide a pretty good corral and squeeze chute, or take the heifers to your veterinarian at least twice. Even when things go well, expect 70% - 80% conception. A good young gentle bull may be your easiest route. The heifers should be ready to breed when they are approx. 70% of their adult body weight. Don't get them too fat as this will decrease conception. a BCS of about 5 or 6 is ideal. You can find examples of BCS scoring all over the internet. You can either sell these animals as heavy bred heifers (they should bring a premium if they are scheduled to calve at the right time of the year) or take a little risk and calve them out and sell as a pair. Keep them a little longer and let them breed back and you can sell them as a pair with the cow bred back. It all depends on how involved you want to be and whether or not you want to try to breed, winter, calve, re-breed, etc. You can stop the process at any time by selling what you have. Then you can re-stock or try something different. Also, the cattle market will fluctuate over the course of the year and you can decide for yourself when to sell. Sale timing may also be dictated by your monetary needs throughout the year.
Regardless of how many, what kind, and how you manage them, eventually you will need to implement a health program involving vaccinations, deworming, fly control, mineral supplementation. To do this properly, it is mandatory that you either have a corral / chute, or a way to catch and haul to your veterinarian.
Good luck, and have fun with your new endeavor!!!