Do you know how the equity is currently divided? Is it 40-40 for the two founders (assuming only 2) and 20% left over? Have they taken any equity funding yet? Is it just starting out, or would you be employee number 13?
Knowing what they have available to offer, where they are along the start-exit line, and their current equity position will give you a good idea of how to negotiate.
Another angle, will you be taking a salary? If you're making a modest salary then it is perfectly acceptable to be offered only a small fraction of equity. You may very well be making more than the founders are.
If you want more equity in the company, offer to forgo a salary or take only what the founders are taking.
working at a startup is riskier for employees than for founders.
I disagree. The founders take the most risk, and get the most reward because of this. Founders typically make millions or $0. Employees don't typically make millions, but it's against the law for them to make 0. They earn a regular salary, which is less than the millions the founders might make, but they earn it consistently. There's a trade off.