Using an 8 speed cassette on an 10-11 speed Tiagra FH-RS400 I'm trying to build a new set of wheels for touring trip (2 months) I have planned and can only find a Tiagra FH-RS400 hub. The issues is that my shifters are Claris 3x8 ST-2403-L and cannot support 10 or 11 speed.

Can I use an 8 speed cassette on the Tiagra hub and if so what size spacer would I need?

Thank you in advance!
The hub for 8 to 10 speed are the same. You are good to go. You should not need any spacer for them.

However, if you mean this hub https://bike.shimano.com/en-US/product/component/tiagra-4700/FH-RS400.html they say it is a 10/11 speed hub and hence you need a (1.85 mm, as mentioned in a comment) spacer for 8 to 10 speed for this one. Those are included by some shops.

Because it is a Shimano hub you can also verify the compatibility in the official chart https://productinfo.shimano.com/#/com?cid=C-453&amp;acid=C-454 

A similar hub which is simply 8/9/10 speed would be (Sora) FH-RS300. It could also be cheaper.