I attempted to buy AVE tickets from Seville to Madrid on-line but could not do it because I don't have a Spain N.I.F. number. We arrive in Spain October 6 and will not be taking the AVE until Sunday, October 15. Do you think we will have trouble making reservations and buying the tickets when we arrive? I don't know how quickly the AVE reservations fill.

Yes, you can buy the tickets there, no problem... that's what I did.

Check for a holiday, but if not, you should have time to purchase in country.

That should pose no problem. I have bought them just a couple of days in advance. Just buy them when you arrive.

Thanks so much for responses. This is helpful information.
Sul

If you still want to buy them online, just write your passport number where it says NIF

Tried the passport number in the N.I.F. space on the Renfe website, but still got "NIF not valid" message. Looks like will just need to get tickets in Spain, which sounds like shouldn't be a problem.

At renfe website you can make a reservation instead of purchasing. The system shall give you a code for buying later at a station, with an expiry date, take a look as it may interest you. You can make as many reservations as you want.
Once in Spain, you can phone 902240202 to make reservations or purchases thru operator, not machine. I do not know if english available.