Can a buyer force me to justify every number in my asking price?

Not in a strict legal sense — you're not obligated to answer every question a buyer asks, and you can decline to negotiate at all. But practically, a serious buyer's due diligence will test the numbers behind your asking price regardless of whether you consider yourself "forced" to answer, because their willingness to pay your price, or to complete the deal at all, generally depends on being satisfied with what's behind it.

The nuance is that refusing to explain your numbers doesn't protect your price; it usually just ends the negotiation, since a buyer who can't get comfortable with what supports your figure will typically walk away or offer significantly less to account for the uncertainty rather than simply accepting your number on faith.

A more useful way to think about it is that you control how much you engage, but the market controls the consequences of that choice. Deciding in advance what level of detail you're comfortable sharing, and at what stage of the process, with guidance from a business lawyer, lets you manage this deliberately rather than either over-disclosing early or stonewalling a serious buyer.