Hi Jalan,
Based on the pictures i’ve seen it appears to have 12 slats and if they are 1x3 (which are actually 2.5" wide) then the gaps between the slats would be a little under 4". Having said that … this may not be correct so they would be a more reliable source of information.
I would keep in mind that there are no “standard” definitions or consensus of opinions for firmness ratings (or firmness numbers) and different manufacturers can rate their mattresses very differently than others so a mattress that one manufacturer rates as being a specific firmness could be rated very differently by another manufacturer. Different people can also have very different perceptions of firmness and softness compared to others as well and a mattress that feels firm for one person can feel like “medium” for someone else or even “soft” for someone else (or vice versa) depending on their body type, sleeping style, physiology, their frame of reference based on what they are used to, and their individual sensitivity and perceptions. There are also different types of firmness and softness that different people may be sensitive to that can affect how they “rate” a mattress as well (see post #15 here) so different people can also have very different opinions on how two mattresses compare in terms of firmness and some people may rate one mattress as being firmer than another and someone else may rate them the other way around. This is all relative and very subjective and is as much an art as a science.
Having said that … based on most of the feedback that I’ve seen the Tuft & Needle feels firmer to most people than most of the other “simplified choice” mattresses (or the medium firmness option for the ones that have multiple firmness choices).
Assuming that you decide to pull the trigger on the Tuft & Needle … congratulations on your new mattress 
I’m looking forward to your comments and feedback when you receive it and have had the chance to sleep on it for a bit.
Phoenix