Does taking methotrexate make your immune system stronger or compromised?
RA and immune system: Does taking methotrexate make... - NRAS
By my understanding at least, if Mtx is the only thing you're taking then you may be slightly more at risk than the general population, but are unlikely to fall into the "high risk" category unless you're also taking other medications in addition and/or have other health conditions to take into account.
Also, as helixhelix has said, and also according to my understanding, uncontrolled RA is likely to put you at more risk than the (relatively?) marginal risk of taking the Mtx so in other words... it's probably best to keep popping those pills if/until your medical advisors tell you otherwise