How many links?

You can count to 1023 on your fingers and thumbs but it makes holding stuff difficult

You can count to 144 on your fingers:

http://www.t2ah.com/2012/03/count-to-60-or-even-144-on-your-fingers.html

or you know, just use your brain! St. Ambrose is documented as the first silent reader in the 4th century AD, counting is an abstract process, you don't need a reference point if you are sufficiently highly evolved.

Another tip is if you get some heavy duty wire hook one end several links in and hook the other several links in on the other side. This will leave both hands free to fit power link or use chain tool with the rivet. The derailleur should move forward sufficiently to allow this.

Counting them is right but what 'giff77' said is the way to do it.

All you need to do is wrap the chain round the large chain ring and the large sprocket. Don't feed the chain through the derailleur. Line up the ends so the chain is taught. Add on two links. Then break the chain. If using SRAM you will have a power link so you should have two inner plates opposite each other to be attached. If Shimano then you will have a outer plate and inner plate ready to be attached with a rivet. Remember to feed shortened chain through dérailleur before joining. Hope this helps.

The chain does stretch but not noticeably, measuring tools range from 1/4, 1/2, 1%. So just count the links on the old chain. If you have a chain splitting tool you can get the longer length and reduce, especially if you have an odd number. Fix up the same number of links and it will be all new, just tighter...

TIGHTER...

Sheldon's and plenty other websites will tell you how to determine right number of links.

But the sure fire way if your current chain was the right length is to count the links yourself. Tedious but reliable, providing door bell doesn't go or small child comes into room and shoves a toy in your face mid count. But how certain are you that your chain was the right length to start? If you aren't then head to Sheldon's...