Only the 250 He-51 inter-belum bi-plane fighters, no other LL to Nat. Spain, such as a small monthly quota of rifles?
I have usually found that you need to send 150 planes of what ever type at least to Spain for them to get up in the air and use them so that you gain the air experience. However often in HOI 4 for some reason you need to give them just over that for it to work out, or even if you send as a 'once' one-off all 280 He-51s that 3 or so get left behind and hold up the whole shipment and you don't actually get any air experience until you finally send the last of them from the initial all out LL 'once' delivery. Since I usually empty my field fighter squadrons by reducing the 4 existing Jagds to just 1 single plane each when the SCW breaks out and then immediately send the LL with the 280 after the last ones arrive in reserve after those singles have been replaced with Me 109s - may be there is a time delay cool-off period for these singles when they arrived in reserve. Perhaps all the He-51s need to have been in reserve for a time period so you do send them all in a 'once' one hit right at the start of the SCW... I might try that from day one.
I like to micro the SCW volunteers and each to their own preferred handling play path, but I like to leave that Bilbao / Basque Republican enclave and all the mountains there alone and instead just fight down in the eastern plains, to avoid the difficult hard fighting terrain so I save on attrition and hopefully by leaving it to the Nationals to ensure that it draws out the SCW a bit more for experience gain...