You could use a negative look-ahead, i.e. only print the number from the "cpus:" line if the colon is not followed by more space-separated digits.

johngg@shiraz:~/perl/Monks$ perl -Mstrict -Mwarnings -E '
open my $inFH, q{<}, \ <<__EOD__ or die $!;
available: 8 nodes (0,8,250-255)
node 0 cpus: 0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 2
+2 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 4
+5 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65 66 67 6
+8 69 70 71 72 73 74 75 76 77 78 79 80 81 82 83 84 85 86 87
node 0 size: 129733 MB
node 0 free: 125997 MB
node 8 cpus: 88 89 90 91 92 93 94 95 96 97 98 99 100 101 102 103 104 1
+05 106 107 108 109 110 111 112 113 114 115 116 117 118 119 120 121 12
+2 123 124 125 126 127 128 129 130 131 132 133 134 135 136 137 138 139
+ 140 141 142 143 144 145 146 147 148 149 150 151 152 153 154 155 156 
+157 158 159 160 161 162 163 164 165 166 167 168 169 170 171 172 173 1
+74 175
node 8 size: 129826 MB
node 8 free: 126395 MB
node 250 cpus:
node 250 size: 16128 MB
node 250 free: 16127 MB
node 251 cpus:
node 251 size: 12032 MB
node 251 free: 12031 MB
node 252 cpus:
node 252 size: 16128 MB
node 252 free: 16127 MB
node 253 cpus:
__EOD__

while ( <$inFH> )
{
    say $1 if m{(?x) ( \d+ ) \s+ cpus: (?! \s* \d) };
}

close $inFH or die $!;'
250
251
252
253
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I hope this is helpful.