Hi
This is a good and common question.
The answer depends on numerous factors. Just to name a few: how complicated the project is, how talented the person is, do you want them solely as a service provider (who then leaves at the end of the project) or as a co-founder long term?
Additionally, it depends on what the Coder wants (and not only what you want).
1. I suggest that you start by trying to "sell the dream" of your idea to potential coders and then once they show interest, ask them how many shares they want. Then, based on the above factors decide how many shares you're willing to give.
2. Whatever the amount is, be sure to:
a. Allocate the shares/right to shares gradually (through vesting) and not all at once.
b. Get a waiver regarding the IP rights and an NDA for confidentiality.
c. Get copies of the progress as the coder advances.

p.s.: if the project is interesting enough *you can send me a pitch/investors deck), I know a programming company that will do the programming (and provide support) for you - for free - in exchange for shares. Your only costs will be the legal costs for drafting the agreements via their lawyer.

I've successfully helped over 300 entrepreneurs, startups and businesses, and I would be happy to help you. After scheduling a call, please send me some background information so that I can prepare in advance - thus giving you maximum value for your money. Take a look at the great reviews I’ve received: https://clarity.fm/assafben-david

Good luck