Before upcoming games, there is another factor, worth to pay some attention. Some national teams will be forced to play in neutral grounds, like we saw in Champions league. For example, Portugal will play in Turin. Also, I haven't checked squads of teams, but I think there will be some missing players who weren't able to arrive because of travel restrictions.
My question is what is the need to take such a risky move? Europe as a whole is going through the third wave of the pandemic and even the preliminary phase of vaccination won't be complete by the time these matches start. I agree that only 5,000 people are going to be seated in a stadium with a capacity of 55,000. So what is the point in allowing so little people? As far as gate collection is concerned, it will be less than the expenses that are required to allow the entry of spectators.
Where is the risk? As you said, there will be just 5000 people in 50 000 seats stadium, so distance between them will be huge. And even with vaccination, virus is likely to stay here for long time, so we have to learn how to live with it. Lockdown can be solution for short time only.
Yes, they won't earn much money from such little number of fans, but even 5000 is better than 0