The ordinal numeral corresponding to 1 is first, so we write 1st.
The ordinal corresponding to 2 is second, so we write 2nd (or perhaps 2d if American).
The ordinal corresponding to 3 is third, so we write 3rd (or AmE 3d).
The ordinal numerals corresponding to 4 and upwards are formed with the suffix th, so we write 4th, 5th, 6th, 7th etc.
But after 20th come 21st twenty-first, 22nd twenty-second and 23rd twenty-third.
When we use algebraic expressions instead of ordinary numerals, the ordinal corresponding to n is nth, pronounced enθ. That corresponding to x is xth, pronounced eksθ. One of the teachers at my secondary school had the nickname kjuːθ, i.e. qth.
But it is not clear how to form ordinals for expressions such as x2, (x+1), and (x-2).
I was quite taken aback when, in the course of reading Seeing Further (ed. Bill Bryson, London: HarperPress for the Royal Society, 2010), I came across the sentence (p. 379)
[May, Oster and Yorke] identified simple features displayed by wide classes of difference equation relating the (n+1)st to the nth state of a system as it made the transition from order to chaos.
Do we really say en plʌs fɜːst? It feels wrong to me.
Imagine there is a queue of people waiting to enter a club. Your job is to pick which ones can go in. You might decide to take person number one (the first person), person number two (the second person), and person number three (the third person). More generally, you might decide to take person number n (the nth person), person number (n+1), (the (n+1)??? person), and person number (n+2) (the (n+2)??? person).
If pushed, I think I’d go for saying en plʌs wʌnθ, en plʌs tuːθ and writing (n+1)th, (n+2)th.
Any mathematicians have views on this?