what is a realistical ads revenue number for a starting blog aiming for 5-10k visitors
It depends on the market, but average CPM (Cost Per Mille) is roughly around $3. So that means for every 1,000 pageviews you can expect about $3 in revenue.
Let's assume you have about 3 pageviews per visitor (look at your analytics to see the real number). At 5-10k visitors that means around 15–30k pageviews or $45–$90 in revenue.
Please note that is a very rough estimate as it really depends on your market, click-through rates, etc. These calculations merely serve to give you a way to make your own estimation.
Assuming those are monthly numbers you should also keep in mind that's probably not worth the overhead for larger advertisers to deal with you. If you're set on monetising at this early stage you might be better of with affiliate programs and get paid per conversion.