How much produce becomes bikkurim (first fruits)?
Rambam says: Hilchos Bikkurim Chap 2
Halacha 19 How should one separate the first fruits? A person descends to his field, sees a fig tree, a cluster [of grapes], and/or a pomegranate tree that has budded, and ties them with a reed.50 He should say: "These are the first fruits.' They become designated as first fruits even though they are attached to the ground after they have been given that title, even though they have not ripened entirely.51
When they ripen and are reaped from the ground, it is not necessary to designate them again. If he did not designate them while attached or call them first fruits and then he reaped them, he should designate them after he reaps them.52
This implies to me that only some of the fruits are bikkurim.
If his entire harvest becomes impure, he should not designate the impure produce as first fruits. Instead, as an initial preference, he should set aside other produce as first fruits for the impure produce. It appears to me that if he does not have other produce, to separate [as first fruits], he should not set them aside to be destroyed.53 Similarly, it appears to me54 that if the first fruits become impure, he should not use them as kindling for a furnace, because they are like objects consecrated for the Temple.55
Halacha 20 When a person set aside his first fruits and they rotted away,56 were taken by others, lost, stolen, or became impure,57 he is obligated to set aside others in place ofthem,58 as [Exodus 23:119] states: '...bring to the house of God your Lord.' This teaches that one is liable to replace them until he brings them to the Temple Mount.59
and now by mentioning the entire harvest I am not sure.