The field you are looking for is pagetotal. pagetotal holds the total number of pages of a work. The standard styles only print pagetotal for @book-like stand-alone works (@book, @collection, @manual, @thesis, ...).
For example from biblatex-examples.bib
@book{companion,
author = {Goossens, Michel and Mittelbach, Frank and Samarin, Alexander},
title = {The {LaTeX} Companion},
date = 1994,
edition = 1,
publisher = {Addison-Wesley},
location = {Reading, Mass.},
pagetotal = 528,
}
If you insist on a comma before the pagetotal, you will need to patch the bibdrivers.
\documentclass[10pt,a4paper]{scrartcl}
\usepackage[utf8]{inputenc}
\usepackage[T1]{fontenc}
\usepackage[ngerman]{babel}
\usepackage{csquotes}
\usepackage[style=ext-authoryear-ibid, giveninits=true, uniquelist=false,
backend=biber, natbib=true]{biblatex}
\usepackage{xpatch}
\newcommand*{\commapagetotal}[1]{%
\xpatchbibdriver{#1}
{\newunit\printfield{pagetotal}}
{\setunit{\addcomma\space}\printfield{pagetotal}}
{}{}}
\forcsvlist{\commapagetotal}{book,booklet,collection,manual,proceedings,report,thesis}
\addbibresource{biblatex-examples.bib}
\begin{document}
\parencite{companion}
\printbibliography
\end{document}

The original question had an @incolletion entry as example. There things are a bit more tricky...
For works like @inbook, @incollection and @article the field is not used because the total number of pages can be easily calculated from the pages field.
There is a slight semantic quirk of filling the pagetotal field of a @in... type work with the total number of pages of the enclosing work, but a similar thing happens to editor or isbn, so...
You need to patch the driver of @incollection to include pagetotal. This can be done with xpatch's \xpatchbibdriver.
\documentclass[10pt,a4paper]{scrartcl}
\usepackage[utf8]{inputenc}
\usepackage[T1]{fontenc}
\usepackage[ngerman]{babel}
\usepackage{csquotes}
\usepackage[style=ext-authoryear-ibid, giveninits=true, uniquelist=false,
backend=biber, natbib=true]{biblatex}
\usepackage{xpatch}
\newcommand*{\addpagetotalto}[1]{%
\xpatchbibdriver{#1}
{\usebibmacro{chapter+pages}}
{\usebibmacro{chapter+pages}\newunit\printfield{pagetotal}}
{}{}}
\forcsvlist{\addpagetotalto}{inbook,incollection,inproceedings}
\usepackage{filecontents}
\begin{filecontents}{\jobname.bib}
@incollection{Pail.2017,
author = {Pail, Roland},
title = {Globale Schwerefeldmodellierung am Beispiel von GOCE},
pages = {217--257},
publisher = {Springer Spektrum},
editor = {Rummel, Reinhard and Freeden, Willi},
booktitle = {Erdmessung und Satellitengeodäsie},
year = {2017},
address = {Berlin},
pagetotal = 543,
}
\end{filecontents}
\addbibresource{\jobname.bib}
\begin{document}
\parencite[220]{Pail.2017}
\printbibliography
\end{document}

I personally think the information is superfluous, but if you like it...
nop, but there ispagetotalfor a similar purpose.pagetotalis however only available for the types@book,@collection,@manual,@reportsince it holds the total number of pages of a single work. The total number of pages of a@incollection,@inbookor@articlecan be calculated from thepagesfield. In your example thepagetotalwould refer to the enclosing@collectionwork whilepageswould refer to the@incollection. I don't thinkpagetotalmakes a lot of sense here. – moewe Jun 24 '18 at 19:41pagetotal– moewe Jun 24 '18 at 19:51