How many saint quartz can I still save for Merlin(F2P)?

https://shipfu.moe/fgo/
you can use that website to roughly estimate how many quartz you will get from logins, weekly master missions, etc

I think that 50 SQ more could help. I'm saving 250 for him

Assuming that Merlin will be the only 5* rate-up servant in a banner, I've typcially seen that the overall odds of pulling him are just 0.7%. Thus, a quick calculation of the probably of not getting him after 100 pulls is 0.993 ^ 100, or 49.5%. So, roughly speaking you have about a 50/50 chance of getting him if you spend 300 SQ. (BTW, the odds of getting *any* 5* after 100 pulls is only 63%...)

So, in other words, save all you can because RNG will never give you any amount of certainty, and the pull rates are abysmal IMHO.

BTW, the above is calculated without regard to the 10-pull guarantee, which is actually not all that likely to trigger, and even if it does, to give you a 5*.