Instead of doing N findOne operations, do a single find operation. Could add some code to explicitly handle 1 vs. N, but it's cleaner to just use arrays for everything, including single elements.

sendMany.js

send(m){
		if (m.numbers){
			m.numbers.forEach((number)=>{
				this._sendSMS(m.text, number);
			})
		} else {
			let sentTo = [];
			Devices.find({userId: {$in: m.users}}).forEach((device) => {
				const devId = this._sendNotification(device, m.title, m.text, m.data);
				devId && sentTo.push(devId)
			})
			Devices.update({_id:{$in: sentTo}},{$set:{lastUsed: new Date()}});
		}
	}