As others have noted its usually a good idea to have a backup plan (an offer letter from another company), before broaching a subject like this.
However I want to ask why, you want your boss to hire someone else (I am guessing it's not that you are lonely and want someone to talk to).
More likely you want someone else to take on some of your work load.
Therefore your real ask is that your boss reduce your workload rather than hire someone else, in fact if your boss did hire someone else and the new hire was incompetent, it may not help you.
Therefore my advice would be to ask for what you actually want (reduced workload) and let your boss figure out how (and if) to accommodate your request. Note, (s)he may go in a different direction such as offering you a pay raise, would you accept that? if so how much would you be looking for?
I would also recommend establishing a timeline for when things will change.
Finally, it may be the case that your boss is happy with the status quo and simply refuses to make any changes, so you should be prepared to make the jump...