Linear Array help
It depends on the elements you're using and so on...
For example if you use patch antenna, each element can be estimated to have anywhere between 5 and 8 dB of gain. So you have a rough estimation, say between 4 (12 - 8) and 7 (12 - 5) dB of gain have to come from the array factor (number of elements).
3 dB is the doubling of the elements, so for example to get 4 dB of gain you'll need 3 elements 10 * log10 (3) = 4.77 dB as an example.
There are also power combiner losses and so on and it's not very easy to estimate the final outcome but it's a start.
Let us know if this helps.