A garden has 2000 trees.

Let the number of orange trees be $x$.

It is given that there are 2,000 trees. While $12 \%$ of them are mango trees, $18 \%$ are lemon trees and the rest are orange trees.

Now, $(12 \%$ of 2000$)+(18 \%$ of 2000$)+x=2000$

$\Rightarrow \frac{12}{100} \times 2000+\frac{18}{100} \times 2000+x=2000$

$\Rightarrow 240+360+x=2000$

$\Rightarrow 600+x=2000$

$\Rightarrow x=2000-600$

$\Rightarrow x=1400$

$\therefore$ There are 1,400 orange tree $s$.