[lojack]

The first thing I'd do is look up their funding on AngelList. This can be an indicator of what the equity is worth -- without knowing the terms it's only an indicator though. These numbers are also useful for determining how stable the company is. If they have 8 engineers and 10-12 other employees but only had a $500k seed round, it's a very risky job -- if its closer to $2m, its a bit safer.

As far as actual numbers go, that's entirely dependent on what you think you're worth, and what you think that equity is worth. You're the 21st employee, and they are raising money, so it isn't exactly a red flag to me. Treat the equity as a bonus that may never come to fruition.