Is there a standard for number of LP's per inch?

Take a few samples from your collection to see how many per inch.   Then use the average per inch X # inches total and there is your estimate.

I'd separate out box sets if you can and count or estimate those separately.

also consider if a double album is one or two records as that will effect your outcome. also jacket thickness to some degree.

Guys thanks for all your responses. I was really trying to see if one of the collector organizations had previously set a standard. I'll take all your tips into account. Best to all.

You know guys can’t judge inches; they always overestimate. 

No standard depends on the thickness of the pressings etc.No such thing as LPs by the inch or foot.Good luck though!!