The first heat in a Pembroke Welsh Corgi female can start between 6 and 12 months of age. From my personal experience 8-9 months. The first heat is profuse and since the bitch is not yet very aware of what is going on, the first time is just learning to groom herself. Subsequent heats are less noticeable. Heat in medium sized breeds tends to occur twice a year. During the heat period, special panties are very helpful. During the entire period of heat (3-4 weeks), it is necessary to walk on a leash to avoid unwanted mating.
There is a myth that a bitch just needs to "give birth once" for her health! Remember, giving birth has never been healthy for anyone, and your dog doesn't want to cuddle with puppies at all.
Oh yeah, your friends and family really want a puppy just from your dog! Believe me, when you have puppies, everyone will start having renovations, moves, divorces, and other joys of life that have no place for a puppy. Even more frustrating for them will be the fact that you want money for a puppy, and you will definitely want it. Mating, giving birth, and raising puppies is expensive financially and morally.
Can a bitch be spayed if you do not plan to breed her? You can, although I personally am against sterilization without medical indications. Neutered bitches gain extra weight very quickly and urine leakage is possible. It is possible to spay a bitch not earlier than 1.5 years.
To summarize, I want to say that each sex has its pros and cons. Also, by the nature of Pembroke males differ a lot from each other, as well as bitches.
Some people are closer to males, as I am, some people are closer to bitches.