It hugely depends on the company because there's no "one size fits all" answer. The standard Alchemist $6mil cap is probably an okay starting point, but it really just depends on the specifics.
One strategy that can be useful is increasing the cap based on how much you've raised so far, so e.g., first $50k gets a $3mil cap, next $50k gets a $4mil cap, then next $100k at $5mil, then the rest of the round at $6mil etc.
The specifics of what those numbers look like will depend on your company and investor interest though.