Budgies are one species that is relatively easy to keep more than one. Once you get a second Budgie, it sometimes seems a good idea to get another one. After all, Budgies come in many different colours.
I once had a flock of approximately 60 Budgies and every one looked at least a little different from every other one. Some of the birds were the same colour as some of the other birds, but then you can take the different sizes into account.
You can have the larger English Budgies and the smaller Australian Budgies as well as a combination of the two when they have been crossbred to get a midsized bird.
You can keep a flock of all male Budgies so you won't have to worry about any eggs or babies, or you can keep a flock of all female Budgies, so that even if eggs are laid, you won't have to worry about babies, or you can keep a mixed flock. With a mixed flock of Budgies, you will normally not have egg laying or babies unless you provide nestboxes for them to use, but occasionally, a pair might decide to start a family anyway.
The main limitations as to how many Budgies you keep is the amount of space you have to keep them along with the size of cage and amount of food and veternary care you can afford. Another point to think about is that one Budgie can chew woodwork as well as books and any items left lying around, so a flock of Budgies can be quite destructive. My flock of 60 Budgies actually ate a good portion of the walls of the aviary that housed them.
Of course, one Budgie makes an absolutely wonderful pet and may learn to talk, whereas a Budgie within a flock will normally not be as tame and will probably not talk at all.
I have a short video of Sweetie the Budgie and since he is new to my home I am still trying to figure out what he says. Please take a look (he's hard to see, but you can hear him) at Hi Sweetie and maybe you can help me figure out his words.
I wrote this Budgie e-book after many years of keeping and breeding these wonderful little birds.