The holders wrapped up their Group C qualifying campaign with a 1-0 victory away to Ukraine on Monday, when Del Bosque had the luxury of fielding an experimental side with Spain already through as section winners.
While the eight defenders and four forwards may be easier to select, among the midfielders who will be pushing for a place at the tournament in France next year are Cesc Fabregas, Isco, Thiago Alcantara, Sergio Busquets, Koke, Andres Iniesta, Santi Cazorla, David Silva, Juan Mata, Javi Martinez, Mikel San Jose and Bruno Soriano.
Most of those players would make it into any national team but four are likely to miss out as Spain chase an unprecedented third consecutive continental title after their triumphs in 2008 and 2012.
"It was good that we were able to look at some different players and test whether they will be of use," Del Bosque told a news conference at Kiev's Olympic Stadium.
"It's going to be a problem choosing 23 players but there is still a long way to go," he added.
"We have a good substance, some good players, and that makes the coach's task easier."
Spain's eighth qualifying win in a row put them on 27 points from 10 matches and they have gone eight competitive games without conceding for the first time.
Slovakia finished with 22 points and Ukraine, who go into the play-offs, with 19, eight ahead of fourth-placed Belarus.
Del Bosque praised goalkeeper David De Gea, who was deputising for captain Iker Casillas and made a host of fine saves to deny Ukraine.
De Gea is the natural successor to Casillas but the former Real Madrid keeper, now at Porto, will almost certainly remain Spain's number one for what would be his final major tournament.
(Writing by Iain Rogers, editing by Martyn Herman)
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