Your benchmark formula is Duration (in days) = Work (in hours) / Utilization (workers). This assumes perfect resource elasticity. There're few tasks with perfect resource elasticity.
If you have a reasonable estimate for the work based on one worker, then use this formula to calculate the same estimate using additional workers. This will establish your floor.
I am also assuming you established your one worker estimate resulting in a range, i.e., best, worst, most likely. If you did not, then you need to do this.
Since tasks are rarely perfectly elastic, your challenge is to arrive at a multi-worker estimate where you have little to no experience. Since you have the floor as established above, you need to arrive at how much "fat" you need to put into this estimate since you are carrying a lot of risk.
You need to consult as many SMEs in this work area as you can find to get an idea of what the "engineering estimates" look like, knowing that this type of estimation has significant validity issues secondary to human biases. You need to research historical data on similar projects if you can find them and use them where you can, understanding that this too has some validity issues such as how accurate is the data in the first place. If you are able to uncover any parametrics with the tasks, use them where you can; however, in knowledge-type work, these don't really exist with any degree of high reliability and validity.
Absent the above, you simply have a lot of risk that you are unable to reduce through knowledge. So you are left with keeping a lot of "fat" in your commitment. This means, your starting point is the worst case value in your one-worker estimate and you reduce very little as you add resources. For example, if you found for a particular work package that it will take 8 days in duration (worst case) to finish using one worker, you may choose to commit to only one day reduction adding a second worker, maybe two day reduction adding a third. There is no science here to back this up, no real basis. But it provides you with reasonable insulation to protect yourself until you know better with experience. The risk, of course, is that there are some tasks where your duration degrades (instead of 7, it climbs to 10) with adding resources. But since your starting point is the worst case value, you have reasonable contingency built in. You communicate high risk and inform your stakeholders early any unfavorable variances you being to accrue and ensure there is contingency in the budget (yes, even more than your fat estimate) to protect the project holistically.
