Update: 06-Jun-2017. This build was for factorio v0.14 and has been superceded by my v0.15 build 'Skynet on Steroids' https://factorioprints.com/view/-Kly_Swi1toD0g2nG8RG
A simple combinator, inserter and requester chest that only adds bots to the network when none are available, but limits the network to a maximum number of bots (set in a constant combinator).
The blueprint is for logistics bots. It's simple to edit the combinators so that it works for construction bots.