As much as you can afford to lose, which is by the logic I followed. Bitcoin is still very volatile. If you're putting fiat into bitcoin as a investment, try to see the longterm rather than possible short term gains. That said, I do have high hopes but do keep the risks in mind before you turn everything into BTC.

Honestly don't even think about how many BTC you get, but rather how much it costs to get you so much if you're investing fiat into it. Probably a bit more healthy way to approach it even if reaching certain barrier of BTC might seem more psychologically satisfying.