Does Siddharta Gautama happen to be the last person to reach Buddhahood?
Yes, if you think in terms of a fully enlightened Buddhas. Maytrea Buddha will be the next. However, there are also private Buddhas. Most of Buddhas are private Buddhas. They are enlightened, but they don't teach the Dhamma.
To add to @konrad01, private Buddhas don't occur when the dispensation of a Buddha is still there. Hence the last Buddha is Siddhartha Gautama.