You can give the item itself a width value of 45%. Also, flatlist has a property called columnWrapperStyle that you can give the value justifyContent: 'space-between.
Heres an example:
<FlatList
columnWrapperStyle={{justifyContent: 'space-between'}}
data={ApiData}
numColumns={2}
renderItem={({item}) => {
return (
<item style={{width: '45%'}} />
);
}}
/>