That depends on the number of escaliers this is about…

In standard French, an escalier is a single suite of steps, just like a ladder is a suite of rungs.

However, there used te be regional usages in France and Belgium¹ where even a single escalier might be used at the plural, so you’ll find, il est tombé dans les escaliers, redescendant les escaliers quatre à quatre (Marcel Proust). Both the singular and the plural can be heard nowadays but the singular is more frequent, as shows Google Ngram viewer:

Note also that there is a fixed expression that almost never uses the plural: la concierge est dans l’escalier.

¹_{Ref. Le Littré and Le français correct : Guide pratique des difficultés
Maurice Grevisse, Michèle Lenoble-Pinson}

Moi j’aurais dis une “marche d’escalier” pour stair

Basically the distinction is that un escalier is a single flight of steps. Les escaliers, however, usually refers to several flights of stairs (or in practice, what an English speakers might refer to as a stairwell, in French la cage d’escalier).

One stair as in a single step is une marche (d’escalier). (Or un degré, but this word is very uncommon in this sense in modern French.)

One stair as in one flight of steps (a series of step from a landing to the next one) is un escalier.

The singular escalier can also refer to a series of flights spanning multiple floors. Whether one uses the singular or the plural in this case
depends on the context. We tend to use the singular when we think of the set of steps connecting one floor to another, even when there are intermediate floors, and to use the plural when thinking of the stairs as a room (the stairwell: la cage d’escalier).

J’ai pris l’escalier pour monter au cinquième étage.   ou   J’ai pris les escaliers pour monter au cinquième étage.
Je suis tombé dans l’escalier.   ou   Je suis tombé dans les escaliers.
Je l’ai croisée dans l’escalier.   ou   Je l’ai croisée dans les escaliers.
* J’ai renversé du café sur la troisième marche de l’escalier.   (Here the plural is impossible since this refers to a specific flight of stairs.)

The distinction between escalier and escaliers is rather fuzzy and depends on the speaker. I’d use the singular in all the sentences above. The *Trésor de la langue française describes the usage of escaliers to mean cage d’escalier as “populaire ou familier”. The 19th century conservative and prescriptive dictionary Littré describes the use of escaliers in the sense of a flight of steps as “erroneous”, but allows its use to mean a series of flights spanning multiple floors.

It’s perfectly idiomatic to keep it simple and always use the singular except when referring to multiple staircases.