What an investor can ask in terms of a percentage of equity or an investment amount depends nearly entirely on variables, so it is impossible to generalize.
Some factors to consider for startup:
Geography (US, LatAm, etc.)
Market (B2B, B2C, Enterprise, etc.)
Sector (Deeptech, Agritech, AI, etc.)
Stage (seed, Series A, etc.)
Size and type of financing round
Runway (cash in bank)
Some factors to consider for investor:
Typical check size
Value-add beyond $$$
Just to contrast, emerging managers of small venture capital funds (say $5M total) may be able to invest $100K into early pre-seed type deals, but may not have leverage to get anything in addition. If we're talking about Sequoia Capital, then they might invest millions at a later stage, get on the startup's board of directors, etc.
Regardless, it's important for founders to maintain control of the company as long as possible to avoid financing risk and optimize for the best outcome - so percentages are often highly negotiated. Founders should take care to model the capital journey, as there are a number of models available and many variables can affect them as the startup gets built.