On 15mg buspar twice a day and 5 mg lexapro microdosing up to 10/15 mg daily. whats my risk for serotonin syndrome?

Even RARE but the risk is there so all you can do is paying attention to your body. If you have any suspicious symptom of this rare syndrome then you stop both medications and seeking medical attention urgently. We could not tell you the exact risk prediction because this adverse effect varies individually and there is no specific risk factor except alcohol which is absolutely contraindicated.