As many as they will let you - there is no set limit. At their discretion.
You'll find that after two or three long visits they may suggest you get a visa for future ones. If they suspect you are a covert resident you will eventually not get in even with a visa.
I have been going every three months or so for the last couple of years, as long as you go back again they don't seem to mind, however sometimes they do comment and ask you questions but as long as it is for genuine reasons it seems to be ok.