It is generally said to ask for that much amount of which you can justify.
Generally, when preparing pitch deck there is always one slide for how much money you want for your venture and in that one need to specify how they are willing to spend that much amount and how will it benefits the business.
There are various reasons for which a particular amount might be needed that also depend upon the phase in which your startup is, some of them are as follows:
1). Acquisition of user base.
2). For Infrastructure need.
3). For managing the operational expenses.
4).For managing the employees.
5). For expanding your business.
6).For increasing it's valuation.
These are certain reasons for which you might need money from investors.
See, no investor will be ready to invest just for sake, they would definitely want to know whether and you are spending, and in how much time you and your team would be able to return their money with maximum profit.
Still the funding part is with the investor and they have also certain limit for investing depending upon your phase, either in Early-stage, or your product is ready, or you are having some traction from the market etc.
VC firms like Sequoia fund startups in early stage through Sequoia Surge program with a funding of 1-1.5 million dollars.
You might have also heard about Thiel Fellowship program in which certain early stage startup is choosen for funding 100k dollars.
Apart from that there is one term sheet which is given by VC firms at different stages of startup to have good operational conditions for the venture.