The answer is simply, as many as you have time for. I want to play my violin, viola, tin whistle, keyboard and mandolin....and I do hope to learn them all eventually....but there just isn't enough time. Learning an instrument takes an incredible lot of time. Right now I think it is absolutely essential that I at least get an hour a day practice on my violin...but I do better when I get two hours a day in two different sessions. Of course that just makes sense. If I get an hour in the morning and an hour after work, I have essentially doubled what I can learn. I don't always get that much practice in though. There are days when I come home from work and I am just plain too tired or my back hurts too much but I do try to put in as much time into practice as I can. If I am not too too tired or I have enough time in I will get out the viola and do a little practice on that and maybe as I am really starting to drag I might do a little piano on my keyboard.
What this means is that progress on the viola and keyboard are very very slow. When I had two weeks off at Christmas some progress was made on the viola and keyboard but after that I couldn't find time for them for a week. I have to get back in the swing of things again.
There are times though that I think, "Other people don't have to come home and practice, they get to just sit and rest after their hard days work....."
But then I never was meant to be like other people....
