I'm currently trying to conceive and my partner smokes marijuana every day, would this cause low sperm count? Partners been smoker 10years +?

Generally not but... If this were a common problem there would be far fewer babies born here in the santa cruz, ca area :) however, there is evidence that heavy marijuana use may lower sperm count somewhat and reduce the ability of sperm to fertilize eggs, but no evidence it causes "infertility" per se. But for couples experiencing fertility issues it would be wise to reduce or stop marijuana use. See comment:.
Sperm count. Marihuana use is known to cause low sperm count. Without a semen analysis is hard to say whether his sperm count is being affected by the use of marihuana. If his sperm count turns out to be low, he needs to stop using and maybe his sperm count will recover, it may take several months.