Why you had to pick on Tau. TAU is my first 40k army back in 2003. I can field 12k in Tau and still have 20 metal stealth suits. Anyway lets see. I have 5k in guard, 300 pts dark eldar, 6k in space marines (all factions), had 10 k in nids which is now 5k due to an accident, used to had 3k in necrons( sold them cause they was too boring), 1k sisters, 2k grey knights, 6k chaos damons and marines, used to have 6k in eldar which I gave to my brother ( still regret doing that). 0h and about 100 points of orcs.
I probably have more or less since I called out rough estimates. I can safely say that I probably have 50k in assorted 40k models with different varieties.
That all been said I am on Tau ban wagon since my army has received a great boost. Same with Daemons. Both armies along with Eldar are mu favorite armies overall to play with, almost forgot to include nids too. Which mean when nids come out I would probably be on that ban wagon too.
With space marines 5th edition killer my list litterly and 6th brought it back which makes me want to play marines.
At times when I am bored I'll play sisters, guard, or grey knights.
All that being said everyone who knows me will know since day one you have a good chance of fighting Tau, Daemons, or Nids.