finding the length of what the chain needs to be is trial and error. Someone else on here might be able to tell how how many links exactly, but I don't know.
Howso?
Cut the chain if it didn't came uncut from the package.
you can see howlong the chain needs to be by installing it around both chainwheels and hold it tight with the axle slammed into the dropouts.
keep your thum where it fits so you know where to cut the restlengt of the chain.
Make sure you install it with the chain on both wheels so it cannot be to short if you made a mistake on cutting the restlength.
Push the pin back in the chain and ride on... sometimes the link you fixed will not be so smooth but will get better in time if oiled/greased well and installed OK.
hope to help you with this... this might not be the best wat but it is the way I do it.