As you can see from this map on the Wals.info site, there are many. More explanations on the main page.
After a quick search I did, it surprised me to see some languages on this list that seemed to use the same word for one and the indefinite article, such as Dutch. I might be wrong, it seems, so you might want to look more in depth for the ones you want to study more.
Anyway, they are 102. I'll list them below for ease of reference alphabetically and ordered according to the continent:
Africa
America
- Hawaiian;
- Lakohta;
- Quileute;
- Pipil
- Rama;
- Squamish;
- Tzutujil;
- Yaqui.
Asia
- Arabic (iraqi);
- Armenian (eastern and western);
- Byansi;
- Cebuano
- Coptic;
- Japanese;
- Mangghuer;
- Nar-Phu;
- Nias;
- Qiang;
- Tajik;
- Thai;
Europe
- Albanian;
- Breton;
- Danish;
- Dutch;
- English (although technically "an" derives from the weak form of Old English ān ‘one.’);
- Frisian;
- Hungarian;
Oceania
