Number formatting: use of digit separators in manuscripts
Digit separators differ between countries, particularly since the decimal seprator is comma in many countries but a period in English/American. Hence in English you may use commas as digit separators while in other countries periods are used. Space is therefore the only separator that is not confusing. A general rule of thumb is to not use separators for single-thousands but start to use them for tens of thousands and up

1000
10 000
100 000
1 000 000

When using spaces in writing, it is good to remember to use non-breaking spaces so that numbers are not broken over lines.