If someone had 100 fruit trees in an orchard, what is the minimum number they could remove before it is no longer classed as an orchard?

The definition of orchard does not include a specific number of trees, only that it is a planting of fruit trees, nut trees, or sugar maples. The plural would indicate at least two. If you were to sell $1,000.00 worth of agricultural products from your orchard during the census year, the USDA would classify your land as a farm for the purpose of the Census of Agriculture. Although they would ask for a close approximation of how many trees you have per acre, it would make no difference if you only had one prolific specimen.

BB,

You asked me to look at this question

The Encyclopedia of Gardening by J C Loudon, published in 1850, has the following: 'An orchard, or a separate plantation of the hardier fruit trees, is a common appendage to the kitchen-garden, where that department is small, or does not contain an adequate number of fruit trees to supply the contemplated demand of the family. As to the size of an orchard, Forsyth* observes 'it may be from one to 20 acres, or more according to the quantity of fruit or the quantitiy of ground that you may have fit for the purpose''
*Forsyth's Treatise on Fruit Trees 1802.

So it looks as if the question is misconceived

Autolycus (GBQ competitor, taking a break, having completed no 92)