How do you get a jpeg with the long side at 10 inches at 72dpi to be 2000 pixels? I'm applying for an art show and that is the specification

Can you type or paste the exact text of the competition size requirements here? Are they requiring the 10 inches, the 2000 pixels, and/or the 72 dpi? Or is that your image starts out 10 inches wide?

Because it’s mathematically impossible to have an image that is 10 inches long, 2000 pixels long, and 72 ppi (dpi). You can have any two of those, but not all three. That’s why Image Size says what it does: It can only give you a mathematically possible answer.

The standard formula for resolution is:

pixels per inch = number of pixels divided by the number of inches

Image Size uses that exact formula.

2000 pixels long side is a very reasonable minimum requirement for, say, a photo competition.

720 pixels is too small for any such purpose! That’s what a web-sized image used to be with older low resolution screens.

You can upsample to 2000 pixels, but I can guarantee it won’t look good. Traditional upsampling will look grainy and mushy. AI upsampling looks immediately impressive, but is in fact very easy to spot because it is too plasticky perfect, with giveaway added details that usually look very artificial and alien.

In short, find another image.

Yea, ​@c.pfaffenbichler said it: 10 inches at 72 ppi is 720px. It's like saying you want 10 feet to fit inside a yard - not going to happen, unless you alter one of the three numbers. It sounds like the competition committee doesn't really know about how image dimensions works. If the end result is a print for showing, then I would keep the 10 inches. Otherwise, I would go with 2000 ppi at 72 and let the inche measurement fall where it may. 

10inches at 72ppi are 720px. 

If you need more then you need to upsample the image which you can do with Image > Image Size. 

But you might get a more aesthetically pleasing result with Image > Generative Upscale, Filter > Neural Filters > Super Zoom, …